Parametric Domain-theoretic models of Linear Abadi & Plotkin Logic

Parametric Domain-theoretic models of Linear Abadi & Plotkin Logic Lars Birkedal Rasmus Ejlers Møgelberg Rasmus Lerchedahl Petersen IT University Technical Report Series TR-00-7 ISSN 600 600 February 00

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Parametric Domain-theoretic models of Linear Abadi & Plotkin Logic Lars Birkedal Rasmus Ejlers Møgelberg Rasmus Lerchedahl Petersen Abstract We present a formalization of a linear version of Abadi and Plotkin s logic for parametricity for a polymorphic dual intuitionistic / linear type theory with fixed points, and show, following Plotkin s suggestions, that it can be used to define a wide collection of types, including solutions to recursive domain equations We further define a notion of parametric LAPL-structure and prove that it provides a sound and complete class of models for the logic Finally, we present a concrete parametric parametric LAPL-structure based on suitable categories of partial equivalence relations over a universal model of the untyped lambda calculus Contents Introduction 3 Outline Linear Abadi-Plotkin Logic PILL Equality 6 Ordinary lambda abstraction 9 The logic 9 Definable relations Constructions on definable relations 3 Admissible relations 3 Axioms and Rules 3 Admissible relations preserved by structure maps 7 6 Extensionality and Identity Extension Schemes 8 3 Proofs in LAPL 0 3 Logical Relations Lemma 0 3 A category of linear functions 3 33 Tensor types 3 Unit object 3 Natural Numbers 6 3 Induction principle 7 36 Types as functors 7 37 Initial algebras 9

38 Final Coalgebras 3 39 Recursive type equations 3 39 Parametrized initial algebras 3 39 Dialgebras 3 393 Compactness 37 LAPL-structures 39 Soundness 9 Completeness Parametric LAPL-structures Solving recursive domain equations in parametric LAPL-structures 6 Concrete Models 6 6 The connection to CUPERs 9 6 Lifting 60 63 Going fibred 6 6 A domain-theoretic model of PILL 6 6 A parametric domain-theoretic model of PILL 63

Introduction In this paper we show how to define parametric domain-theoretic models of polymorphic intuitionistic / linear lambda calculus The work is motivated by two different observations, due to Reynolds and Plotkin In 983 Reynolds argued that parametric models of the second-order lambda calculus are very useful for modeling data abstraction in programming [3] (see also [8] for a recent textbook description) For real programming, one is of course not just interested in a strongly terminating calculus such as the secondorder lambda calculus, but also in a language with full recursion Thus in loc cit Reynolds also asked for a parametric domain-theoretic model of polymorphism Informally, what is meant [] by this is a model of an extension of the polymorphic lambda calculus [, 7], with a polymorphic fixed-point operator such that types are modeled as domains, the sublanguage without polymorphism is modeled in the standard way and is the least fixed-point operator for the domain ; the logical relations theorem (also known as the abstraction theorem) is satisfied when the logical relations are admissible, ie, strict and closed under limits of chains; 3 every value in the domain representing some polymorphic type is parametric in the sense that it satisfies the logical relations theorem (even if it is not the interpretation of any expression of that type) Of course, this informal description leaves room for different formalizations of the problem Even so, it has proved to be a non-trivial problem Unpublished work of Plotkin [0] indicates one way to solve the problem model-theoretically by using strict, admissible partial equivalence relations over a domain model of the untyped lambda calculus but, as far as we know, the details of this relationally parametric model have not been worked out in detail before (We do that here) In loc cit Plotkin also suggested that one should consider parametric domain-theoretic models not only of polymorphic lambda calculus but of polymorphic intuitionistic / linear lambda calculus, since this would give a way to distinguish, in the calculus, between strict and possibly non-strict continuous functions, and since some type constructions, eg, coproducts, should not be modeled in a cartesian closed category with fixed points [8] Indeed Plotkin argued that such a calculus could serve as a very powerful metalanguage for domain theory in which one could also encode recursive types, using parametricity To prove such consequences of parametricity, Plotkin suggested to use a linear version of Abadi and Plotkin s logic for parametricity [] with fixed points Thus parametric domain-theoretic models of polymorphic intuitionistic / linear lambda calculus are of import both from a programming language perspective (for modeling data abstraction) and from a purely domain-theoretic perspective Recently, Pitts and coworkers [9, ] have presented a syntactic approach to Reynolds challenge, where the notion of domain is essentially taken to be equivalence classes of terms modulo a particular notion of contextual equivalence derived from an operational semantics for a language called Lily, which is essentially polymorphic intuitionistic / linear lambda calculus endowed with an operational semantics In parallel with the work presented here, Rosolini and Simpson [] have shown how to construct parametric domain-theoretic models using synthetic domain-theory in intuitionistic set-theory Moreover, they have shown how to give a computationally adequate denotational semantics of Lily In the present paper we make the following contributions to the study of parametric domain-theoretic models of intuitionistic / linear lambda calculus: We present a formalization of Linear Abadi-Plotkin Logic with fixed points (LAPL) The term language, called PILL for polymorphic intuitionistic / linear logic, is a simple extension of Barber and Plotkin s calculus for dual intuitionistic / linear lambda calculus (DILL) with polymorphism and fixed 3

points and the logic is an extension of Abadi-Plotkin s logic for parametricity with rules for forming admissible relations The logic allows for intuitionistic reasoning over PILL terms; ie, the terms can be linear but the reasoning about terms is always done intuitionistically We give detailed proofs in LAPL of consequences of parametricity, including the solution of recursive domain equations; these results and proofs have not been presented formally in the literature before We give a definition of a parametric LAPL-structure, which is a categorical notion of a parametric model of LAPL, with associated soundness and completeness theorems We show how to solve recursive domain equations in parametric LAPL-structures by a simple use of the internal language and the earlier proofs in LAPL We present a detailed definition of a concrete parametric LAPL-structure based on suitable categories of partial equivalence relations over a universal model of the untyped lambda calculus, thus confirming the folklore idea that one should be able to get a parametric domain-theoretic model using partial equivalence relations over a universal model of the untyped lambda calculus We remark that one can see our notion of parametric LAPL-structure as a suitable categorical axiomatization of a good category of domains In Axiomatic Domain Theory much of the earlier work has focused on axiomatizing the adjunction between the category of predomains and continuous functions and the category of predomains and partial continuous functions [6, Page 7] here we axiomatize the adjunction between the category of domains and strict functions and the category of domains and all continuous functions and extend it with parametric polymorphism, which then suffices to also model recursive types In the technical development, we make use of a notion of admissible relations, which we axiomatize, since admissible may mean different things in different models We believe our axiomatization is reasonable in that it accommodates several different kinds of models, such as the classical one described here and models based on synthetic domain theory [6] The work presented here builds upon our previous work on categorical models of Abadi-Plotkin s logic for parametricity [], which includes detailed proofs of consequences of parametricity for polymorphic lamdba calculus and also includes a description of a parametric completion process that given an internal model of polymorphic lambda calculus produces a parametric model It is not necessary to be familiar with the details of [] to read the present paper (except for Appendix A of [], which contains some definitions and theory concerning composable fibrations), but, for readers unfamiliar with parametricity, it may be helpful to start with [], since the proofs of consequences of parametricity given here are slightly more sophisticated than the ones in [] because of the use of linearity In subsequent papers we intend to show how one can define a computationally adequate model of Lily and how to produce parametric LAPL-structures from Rosolini and Simpson s models based on intuitionistic set theory [] (this has been worked out at the time of writing [6]) and from Pitts and coworkers operational models [] (we conjecture that this is possible, but have not checked all the details at the time of writing) As a corollary one then has that the encodings of recursive types mentioned in [] and [] really do work out (these properties were not formally proved in loc cit) We will also extend the parametric completion process of [] to produce a parametric LAPL-structure given a model of polymorphic intuitionistic / linear lambda calculus, see [] Outline The remainder of this paper is organized as follows In Section we present LAPL, the logic for reasoning about parametricity over polymorphic intuitionistic / linear lambda calculus (PILL ) In Section 3 we give

detailed proofs of many consequences of parametricity, including initial algebras and final coalgebras for definable functors and recursive types of mixed variance In Section we present our definition of an LAPL-structure, and we prove soundness and completeness with respect to LAPL in Sections and, respectively The definition of LAPL-structure builds upon fibred versions of models of intuitionistic / linear logic [3, ] In our presentation we assume that the reader is familiar with models of intuitionistic / linear logic In Section we present our definition of a parametric LAPL-structure and prove that one may solve recursive domains equations in such In Section 6 we present a concrete parametric LAPL-structure based on partial equivalence relations over a universal domain model To make it easier to understand the model, we first present a model of PILL (without parametricity) and then show how to make it into a parametric LAPL-structure We also include an example of calculations in the concrete model Linear Abadi-Plotkin Logic In this section we define a logic for reasoning about parametricity for Polymorphic Intuitionistic Linear Lambda calculus with fixed points (PILL ) The logic is based on Abadi and Plotkin s logic for parametricity [] for the second-order lambda calculus and thus we refer to the logic as Linear Abadi-Plotkin Logic (LAPL) The logic for parametricity is basically a higher-order logic over PILL Expressions of the logic are formulas in contexts of variables of PILL and relations among types of PILL Thus we start by defining PILL PILL PILL is essentially Barber and Plotkin s DILL [] extended with polymorphism and a fixed point combinator Well-formed type expressions in PILL are expressions of the form: where is built using the syntax " #$ and all the free variables of sigma appear on the left hand side of the turnstile The last construction binds, so if we have a type % then we may form the type & ' % )(+*,- (/0- $ )( We use,,, 3,, to range over types The list of s is called the kind context, and is often denoted simply by or Since there is only one kind this annotation is often omitted The terms of PILL are of the form: %6 76 8 6 976 : : <; ' To aid readers unfamiliar with these matters, we have written a short technical note containing detailed definitions and propositions needed here []

( where the (,, and are well-formed types in the kind context 6 The list of s is called the intuitionistic type context and is often denoted 6, and the list of s is called the linear type context, often denoted No repetition of variable names is allowed in any of the contexts, but permutation akin to having an exchange rule is Note, that due to the nature of the axioms of the to-be-introduced formation rules, weakening and contraction can be derived for all but the linear context The grammar for terms is: ; 6 "6 ; ; ; %;) ; ; 7; ; 6 let < ' ; ; 6 be in let ; ; be in let ; ; be in We ; use, which bear some graphical resemblance to, to denote linear function abstraction And we use,, to range over terms The formation rules are given in Figure 8 is considered well-formed if for all types appearing in and, is a well-formed type construction and are considered disjoint if the set of variables appearing in is disjoint from the set of variables appearing in We use to denote an empty context As the types of variables in the let-constructions and function abstractions are often apparent from the context, these will just as often be omitted What we have described above is called pure PILL In general we will consider PILL over polymorphic signatures [9, 8] Informally, one may think of such a calculus as pure PILL with added type-constants and term-constants For instance, one may have a constant type for integers or a constant type for lists lists We will be particularly interested in the internal language of a PILL model (see Section ), which in general will be a non-pure calculus We will also sometimes speak of the calculus PILL This is PILL without the fixed point combinator Equality The external equality relation on PILL terms is the least equivalence relation given by the rules in Figure The definition makes use of the notion of a context, which, loosely speaking, is a term with exactly one hole in it Formally contexts are defined using the grammar: 6 let be ; in let ; be in ;, ; let be ; 6 in let ; be in 6 ; %; 6 let be ; 6 ; in let be in A 8 8 context is a context such that for any well-formed term 8 ;, the term 8 ; ' is well-formed A context is linear, if it does not contain a subcontext of the form We prove a couple of practical lemmas about external equality Lemma Suppose 8 are terms such that Then &76 8 6 6 Proof Using the rules for external equality, we conclude from the assumption that 8 " 6 let 6 be in 6 let 6 be in 6

8 8 8 8 8 8 $ & &76 8 6 8 6 8 8 8 8 6 8 8 8 8 ; 8 ; 8 8 $ 8 $ <6 <; ; 76 ; ' <;, <; 7; <; ' ; 8 8 76 " <; ' 8 6 let be ; ' in, 76 8 <; 8 6 ) let be ; ' in 8 ; 8 8 let ; be in disjoint disjoint is well-formed disjoint disjoint Figure : Formation rules for terms 7

8 8 8 8 8 8 8 8 8 8 6 7; ; 7; ; 6 +; 6 ; 6 +; ; let be in ;& ; -term -term -type -type let ; be in ; 6 let be ;& ; in 6" 8 <;& 8 6 let ; 6 be in ; 6 let ; ; be in 6 let 6 be ; in 6 ; is a 8 8 ; ' is a linear context let ; be in let 8 be ; in context is a linear context and does not bind 6" or contain them free 8 6 let ; be in 6 let ; be in 6 is linear and does not bind or contain it free 8 6 ; let be in 6 ; let be in 8 ) 8 < & Figure : Rules for external equality 8

8 8 <; 8 <;& 3 Figure 3: Types of judgements 9 and further that Thus 8 " ) 6 let be in 6& 6 let be 6 in 8 and hence, &, Ordinary lambda abstraction " ) " We encode ordinary lambda abstraction in the usual way by defining and '6 7; where is a fresh variable This gives us the rule For evaluation we have the rule and the equality rules give 8 8 Note that using this notation the constant, 6 let be ; in &76 8 <; '6 7; <; 8 8 6 7; ; 6 ; can obtain the more familiar looking type The logic As mentioned, expressions of LAPL live in contexts of variables of PILL PILL The contexts look like this: " # " 9$ # "%0 &&' and relations among types of " 9% : &&' where 8 ( are well-formed types in context, for all ( The list of s and % s is called the relational context and is often denoted As for the other contexts we do not allow repetition, but permutation of variables The s and the % s are interchangeable ( ( is a context of PILL and the ( 9 : :

The concept of admissible relations is taken from domain theory Intuitively admissible relations relate to as expressed by axiom 8 described later It is important to note that there is no linear component in the contexts the point is that the logic only allows for intuitionistic (no linearity) reasoning about terms of PILL, whereas PILL terms can behave linearly Propositions in the logic are given by the syntax: +;& +; 6 # % &&' ' 6 # % &&' where is a definable relation (to be defined below) The judgements of the logic are presented in figure 3 In the following we give formation rules for the above Remark Our Linear Abadi & Plotkin logic is designed for reasoning about binary relational parametricity For reasoning about other arities of parametricity, one can easily replace binary relations in the logic by relations of other arities In the case of unary parametricity, for example, one would then have an interpretation of types as predicates See also [6, 7] We first have the formation rule for internal equality: 8 <; ; 8 #3 Notice here the notational difference between ; and ; The former denotes external equality and the latter is a proposition in the logic The rules for, and are the usual ones, where denotes implication, are propositions in any context We use for biimplication We have the following formation rules for universal quantification: The side condition 76 3 6 3 $ & 3 # 3 % & #3 #%&&' 3 3 3 is well-formed is well-formed means that all the types of variables in variables in are well-formed in (ie, all the free type variables of the types occur in ) There are similar formation rules for the existential quantifier, we discuss definable relations Before we give the formation rule for +;" and of relation 0

Definable relations Definable relations are given by the grammar: +6 " Definable relations always have a domain and a codomain, just as terms always have types The basic formation rules for definable relations are: $ & # 76 " 3 +6 " Notice that in the second rule we can only abstract intuitionistic variables to obtain definable relations In the last rule, & is an admissible relation, to be discussed below The rule says that the admissible relations constitute a subset of the definable relations An example of a definable relation is the graph relation of a function: +6 " 6 for The equality relation eq is defined as the graph of the identity map If is a definable relation, and we are given terms of the right types, then we may form the proposition stating that the two terms are related by the definable relation: 8 <; +;" () ; We shall also write for +; We introduce some shorthand notation for reindexing of relations For and, we write for the definable relation +6 " 60 Constructions on definable relations In this subsection we present some constructions on definable relations, which will be used to give a relational interpretation of the types of PILL If and + 3, then we may construct a definable relation " " defined by 6 ' +6" 60 If $ & #

is well-formed and is well-formed, ', and $ &' $ " $ ' we may define as $ &' +; $ $ & & ' +; For, we seek to define a relation First we define for any type the proposition on as 6 +6 The intuition here is that types are pointed, and 6 is thought of as 6 Since we have also fixed points, we may think of types as domains, We further define the map as 6 ) let 6 be in 6 76 We can now define +6 " ) 76 +6 Following the intuition of domains, is to be thought of as lifting, and 60 the unit providing the unlifted version of an element The intuitive reading of 6 is, that is related to (represented by the fact, that is related to if neither 6 nor ) and that two ed elements are related if their un- -ed versions are Next we define the tensor product of and for adjoint to # " " # and The basic requirement for this definition is that should become a left in the category of relations "$# to be defined in Section To give a concrete definition satisfying this requirement, we take a slightly long route We first introduce the map < $ < defined as Then we define 6 6 6 let < 6 &% < % 6 6 be in & $ &' & " or, if we write it out, +6 < " $ & ; 7; 3 ' +; 7; 6 6 6 ; 6 6 let be in let be ; in The reason for this at first sight fairly convoluted definition, is that we will later prove, using parametricity, that $ is isomorphic to, and we already have a relational interpretation of the latter The idea of using this definition of is due to Alex Simpson We use the same trick to define a relation on : Following the same strategy as before, we define a relation ('*),+ ' using the map ' $ defined as 6 let 6 6 76 be in -/, where -/ and define 0')+ $ & $ " which, if we write it out, is +6 '" & $&' "3 63 let be 6 in 3 let be in

3 Admissible relations The relational interpretation of a type with free variables is a function taking relations and returning a new relation However, we will not require that this function is defined on all vectors of relations, but only that it is defined on vectors of admissible relations On the other hand this function should also return admissible relations Since admissible might mean different things in different settings, we axiomatize the concept of admissible relations The axioms for admissible relations are formulated in Figure In the last of these rules is a 6" +60" shorthand for +60" Proposition 3 The class of admissible relations contains all graphs and is closed under the constructions of Section Proof Graph relations are admissible since equality relations are and admissible relations are closed under reindexing For the constructions of Section, we just give the proof of We must prove that for admissible relations is admissible &&' &&' 76" 6 & ' &76 " +60" 3 &76 " +60" 6 " 6 " +6" 60 &&' " < where in the top deduction on the left, we have reindexed along the evaluation maps 6 Now, finally, we may give the last formation rule for definable relations: Observe that 9 &&' 3 " 9 & ' 3 & " is a syntactic construction and is not obtained by substitution as in [] Still the notation might be more complete, but this quickly becomes overly verbose In [] is to some extent defined inductively on the structure of, but in our case that is not enough, since we will need to form for type constants (when using the internal language of a model of LAPL) We call the relational interpretation of the type Axioms and Rules The last judgement in figure 3 has not yet been mentioned It says that in the given context, the formulas collectively imply We will often write for 9 Having specified the language of LAPL, it is time to specify the axioms and inference rules We have all the usual axioms and rules of predicate logic plus the axioms and rules specified below Rules for substitution: Rule 76 ; 6 <; 3

$&' & eq & ' 8 <; 6" +6 " +; 6 & 60" +6 " +60" +6" &&' & 60" +6 " 76 +6" & 60" +6 ) " +6 &' & 60" +6 3" 76 &' 3 #3 6" +6 " +6 &&' 60" +6 " &&' 3 60" +6 " +60" &&' 6" +6 " +6" &' 63 &&' 6" +6 " "3 +6" &&' $&&' & 60" +6 " & ' +6" &' $# & & 60" +6 " +6" &' " &&' Figure : Rules for admissible relations

Rule $ Rule 6 The substitution axiom: Rules for -quantification: %&&' Rule 7 % Axiom 8 676 " # +6" 6 6 +6 " Rule 9 Rule 0 Rule Rule Rules for -quantification: Rule 3 ' ' ' ' ' ' ' &76 # ' ' ' ' ' 6 $# 3 ' ' ' ' ' ' ' ' ' # 3 % 3 ' ' ' ' ' ' ' ' ' % 3 Rule Rule Rule 6 External equality implies internal equality: ' ' ' ' ' ' ' 76 ' ' ' ' ' 6 $ 3 '' ' ' ' ' ' ' ' # 3 %&' 3 ' ' ' ' ' ' ' '' ' % 3 Rule 7 8 <;& <; There are also obvious rules expressing that internal equality is an equivalence relation Intuitively admissible relations should relate to and we need an axiom stating this In general, we will use as the test for 6

Rule 8 " & ' 6" 6 " 6 60 6, " 76 +6" +60" We have rules concerning the interpretation of types as relations: Rule 3 Rule 9 Rule 0 Rule ( ( Rule $ $ Rule ( &&', ' & ', 0'*)+ & $& ' Here is shorthand for 6" 76& 6 If the definable relation +6 is of the form " +60" +;", then 6" ; substituted by : 76 " 3 8 <; Rule +6 " +; ;" 6" Axiom 6 8 $ $ should be equivalent to Given a definable relation we may construct a proposition +60" On the other hand, if is a proposition containing two free variables 6 and, then we may construct the definable relation +60" The next lemma tells us that these constructions give a correspondence between definable relations and propositions, which is bijective up to provable equivalence in the logic Lemma 7 Suppose is a proposition with at least two free variables 6 " Then +6 " +6" with Suppose # is a definable relation, then +6 " +6" Proof The first biimplication follows trivially from Rule For the second, we need to prove 3 3 +6 " +60" 3 " which is trivial by the same rule 6

The substitution axiom above implies the replacement rule: Lemma 8 ; ; Proof Consider the definable relation 63 ; 6 6 &76 8 ; 6 3 6 Clearly +;"7; holds, so by substitution +; 7; holds Lemma 9 +6" +6 " +6 6 " Proof Suppose +60" +6 " and that < +; 7; Then clearly +; 6 6 7; and thus, since let 6 6 be 6 6 in ;6 6 ; 6 6 we conclude +6 #6+" Lemma 30 For 6, 6 always holds in the logic Proof Define < as 6 let be 6 in Then clearly 6 Lemma 3 For any #, 6 " 6 6 " Proof Since 6 & let be 6 in 6 this follows from Lemma 30 Admissible relations preserved by structure maps We now proceed to show a couple of practical lemmas expressing that various structure maps preserve admissible relations The maps that we are interested in are, < which, categorically in the models of PILL, are the structure maps of a comonad, and,, which are the maps that make the comonad into a linear category The maps are syntactically given as 6 ) let 6 be in 6 ) let 6 be in 6 ) let 6 be in 6 ) let 6 be in Lemma 3 For all admissible relations &&', ' ) ' ) are maps of relations, ie, +6" implies 60 7 and 6

Proof The lemma clearly holds in the case of 60" 6) of the form Since 60 60 and both define admissible relations from to, by Rule 8 we conclude that +60" implies +6 6 and +6 6 Since +60" +6 we are done Lemma 33 For all admissible relations &&', ) 3 ) 0'*),+ are maps of relations, ie, +6" 3 implies 6 and &')+ 6 is a map of relations, since Proof To prove that let 6 #6 be let be 6 in ' in ; 6 6 let be 6 in ;0 we need to prove that +60" $&&' ; 0 7; ) < ; ; let 3 be 6 in ; 3 3 let 3 be in ; 3 3 Since the expression on the right hand side of the first 6 is admissible 6 in 6" and +60" 6, by Rule 8 it suffices to prove the implication in the case " In this case, let 3 6 ; be in ; 6 6 3 3, so the implication is trivial To prove that is a map of relations, we need to prove that +60" $&&' "3 3 let be let 6 be in in 3 let be let be in in The implication clearly holds in the case of 60" of the form 6, and so, since +6" 6, as before we conclude from Rule 8 that the implication holds in general 6 Extensionality and Identity Extension Schemes Consider the two extensionality schemes: 6 7; 6 7; 6 ;& ; These are taken as axioms in [], but we shall not take these as axioms as we would like to be able to talk about models that are not necessarily extensional Lemma 3 It is provable in the logic that 6 6 6 6, +6 & +6 6 6 In particular, extensionality implies & 6 Proof This is just a special case of Rule 8 8

The schema ' eq eq is called the identity extension schema Here ranges over all types, and eq is short notation for eq 9 eq 9 For any type we can form the parametricity schema: where, for readability, we have omitted after $ Proposition 3 The identity extension schema implies the parametricity schema eq " Proof The identity extension schema tells us that $ $ eq Writing out this expression using Rule 3 for the relational interpretation of polymorphic types, one obtains the parametricity schema In the case of second-order lambda-calculus, the parametricity schema implied identity extension for the pure calculus, since it provided the case of polymorphic types in a proof by induction It is interesting to notice that this does not seem to be the case for PILL, since it seems that we need identity extension to prove for example eq eq eq Lemma 36 Given linear contexts and, suppose 6 6 6 then "3 6 let be 3 in 6 let 6 be 3 in 6 Proof Consider "6 6 6 ' 6 then If 3 then by identity extension eq which proves the lemma eq 3 63 By definition of eq let 6 6 be 3 in 66 let 6 6 be 3 in 66 eq we have This completes our presentation of LAPL In the following section we show how to use the logic to prove various consequences of parametricity We shall write using extensionality and using identity extension to mean that we assume the extensionality schemes and the identity extension schema, respectively 9

8 6 8 3 Proofs in LAPL 3 Logical Relations Lemma Lemma 3 In pure LAPL, for, and, &( Proof Simple induction on the structure of The cases and are trivial For the case 3 3, 33 3 3 $ Likewise for the cases of and The last case is and in this case is not free in, so $ ) 3 ) $& ' 3 $ % $&&' $ $ Lemma 3 Suppose Then 63 8 ; 3 and 63 8 <; 3 " &&' 3 6 " +6" +; +6"7; 6, " +6" let 3 6 ; be in 3 " 3 let be ; 3 in Proof Consider the special case of Rule 8 used on the relations +6 " 0 +6" and +6, " let 3 be 6 in ; 3 " let 3 be in ; 3 This gives us 6 " 6 let 3 6 ; be in 3 " let 3 be ; 3 & in 6, " +6" +6 let 3 6 ; be in 3 " 3 let be ; 3 in From this we conclude the desired implication using the fact that 6 +6" (Lemma 3), let 3 6 ; be in 3 & ; +6 and +6" 6 ; Lemma 33 (Logical Relations Lemma) In pure LAPL, for any closed term In words, any closed term of closed type, is related to itself in the relational interpretation of the type ; 3; and, Proof We will prove that for any term 6 8 6 &; 6 6 in the pure calculus; the proposition & ' 6 6 " 6 ; 6 6 ; 0 "

6 6 6 6 6 6 holds in the logic Here is short for 6 6 The special case of the vectors ; of length zero is the statement of the lemma The proof proceeds by structural induction on, and it is for the induction we need the seemingly stronger induction hypothesis described above ; 6 ( Case ; 6 : In this case ( Case : In this case ; Case 3 : We always have Case ; : This is Axiom 6 is of length one, and the proposition is trivial is empty, and this case is also trivial ; Case 6 0 0 7; ; : By induction, the proposition holds for We must show that if 6, then ; 6 6 0 ; 6 0 The induction hypothesis tells us that if further 0 0, then ; 6 676 0 ; " 0 " ; and since 6 6 60 ; 6 6076 0 we have the desired result Case ; ; ; : ; 7; By induction the proposition holds for the terms, and so since ; 6 6 ; 6 ; 6 3 the proposition holds by definition of Case ; ; ; : ; 7; By induction, the proposition holds for Clearly ; 6 6 & ; 6, ; 6 and so the proposition holds by Lemma 9 : We must show that if Case ; : 0 7; ; 6 6 6 $ : 0 ie, for all : 0 : 0 $ : 0 : 0 : 0, ; 6 6 : 0 $ : 0 ;, then ; 3 " : 0 By induction, the proposition holds for ; But up to the position of the quantifiers : 0 : 0 $ : 0 & ' : 0 : 0 " this is exactly the proposition we need, and the rest of the proof is just simple logic ; ; Case ;- ; : By induction, the proposition holds for So since 6 6 ; 6, then ; 6 6 ; By Lemma 3, we get the desired result 6+ 6, if

3 6 6 ; ; 6 Case : In this case ; Since 6 & ; is of length zero By induction, under the usual assumptions, 6, we need to show ; which follows from Lemma 3 Case ; let 3 3 and if further 3 The latter tells us that 3 63 7; so by definition of as desired be ; in ; : and 3 Case ; let 3 be ; in ; : Case 6 let 3 let ; 6 6 " ; ; We know by induction that ; 6 6 then ; 6 6063 63 6063 63, we get 3 By definition ; 6 6 Suppose we are given 3, then by induction ; 6 ; ; " " 7; ; be ; 6 6 ; in 6 6063 63 be ; in + let 3 ; be 6 6, and suppose 663 ; in ; 63 By Lemma 3 we conclude that if then let 3 ; be in 6 663 ; Since by induction 6 6 " ;, we are done let be in : By induction, if 6 and 6 6 6 663 6 and let 3 ; be in then " If we further know 63 and 6 The definition of the latter tells us exactly that 6 0'*),+ let be 6 6 in 6 6 let be in as desired

8 8 3 A category of linear functions At this point we wish to show certain types definable via polymorphism To state this precisely, we introduce for each kind context the category as follows: Objects are closed types ' Morphisms relation on these terms being internal equality are equivalence classes of terms of type ; the equivalence Composition in this category is given by lambda abstraction, ie yields 6 6 We now aim to prove that under the assumption of identity extension and extensionality, for all types we have an isomorphism of objects of : $ & for not free in We can define terms $ & composed with and by $ & 6 % % 6 and Clearly 6 $ & 76 6 6 so -/ Notice that this only involve external equality and thus we did not need extensionality here Proposition 3 Using identity extension and extensionality, one may prove that is internally equal to the identity Proof For a term -/ 6 $ & we have % % Using extensionality, it suffices to prove that % % holds in the internal logic By the parametricity schema we know that for any admissible relation eq If we instantiate this with the admissible relation %, we get Since -/ as desired eq % % we know that - eq % & % % % -, ie, % % -/ 3 -/ - % &&' 3

6 ( ( ( This proof may essentially be found in [] Intuitively, what 6 happens here is that $ is a subtype of $, where the inclusion maps to application at We use parametricity to show that & does not contain anything that is not in 33 Tensor types The goal of this section is to prove $ for and types in the same context The isomorphism is in the category This isomorphism leads to the question of weather tensor types are actually superfluous in the language The answer ; is yes in the following sense: Call the language without tensor types (and ) and the language as is Then there are transformations ; ; and (, ( being the inclusion, such that and ( settle for the isomorphism above We can construct terms This is all being stated more precisely, not to mention proved, in [, 3] In this paper we < $ < and $ & by and where the map pairing 6 6 let be % % 6 6 in < pairing is "6 6 76 #6 pairing Let us show that the composition is the identity 6 6 6 6 let be % < % 6 6 in let be in % % 6 6 pairing % 6 6 6 let 6 be in % 6 6 pairing let be 6 6' in Proposition 3 Using extensionality and identity extension one may prove that the composition $ & $ & is internally equal to the identity Proof We compute 6 6 & pairing let be pairing % % 6 6 in

Suppose we are given a type and a map % We can define as let 6 6 be in % 6 6 Then 6 6 pairing % 6 6, which means that pairing eq eq % schema % " $ & eq eq & so ie, Writing this out we get pairing % " pairing & % 6 % 6 " $ & let be pairing in % 6 6 % % Using extensionality we get let be pairing in & This is enough, since by the rules for external equality the left hand side is 6 6 let be pairing % % 6 6 in By the parametricity 3 Unit object The goal of this section is to prove that identity extension together with extensionality implies $ The isomorphism holds in for all We first define maps ' $ and $ & as 6 let 6 be in -/ ; $ 7;, where - We first notice that +6 let 6 be in & -/ " let 6 6 be in let be 6 in -/ Proposition 36 Using identity extension and extensionality, we have that identity on $ is internally equal to the

Proof First we write out the definition -; $ let be +; in -/ ; We show that for any $, for any type 6, and any +; we have 6 ; 6 Given 76 as above, we can define % < as % 3 let be 3 6 in Then % is admissible, so by identity extension +; % % +; 6, and by definition Since % 6 % +;, & we have ; % +; & let +; be 6 in let +;, be in - let +; be 6 +; 6 in -/ 6 3 Natural Numbers $ We define the type of natural numbers as, as 6 760 6 6 We further define terms and prove that is a weak natural numbers object in each extensionality, an honest natural numbers object Suppose we are given a type, a term and a morphism as % & Then clearly %, and % 6 & +6 & numbers object We can express the weak natural numbers object property as: for all commutes Lemma 37 Identity Extension and extensionality implies 6-6 <6 Proof Suppose we are given and define % as above Since % & % % and 6 +6, by parametricity of, % +6 +6 6 Letting range over all types and as desired, and, using parametricity and We can then define % % +6, so is a weak natural, there exists an % such that % %, ie, and % over all terms, using extensionality and Lemma 3, we have 6 6, we have 6

We can now prove that is a natural numbers object in each Lemma 38 Assuming identity extension and extensionality, given up to internal equality the unique % such that % &, % 6& % 6 Proof Suppose % satisfies the requirements of the 6 lemma Then % % 6 and reformulation of the requirements), so for arbitrary, by parametricity of, 6 % +6 & % +6 Thus, by extensionality, % % 3 Induction principle, the map % defined as above is % (this is just a The parametricity principle for the natural numbers implies, that if &', and 6, then +6 +6 So if and 6 By Lemma 37, +6 " +6 36 Types as functors 6 Definition 39 We say that free variables, then +6 +6, so we can conclude that +6076 If is a proposition on such that is admissible, then from parametricity we obtain the usual induction principle 6 +6 +6 6 +6 is an inductively constructed type, if it can be constructed from " ' $ and closed types using the type constructors of PILL, ie, and For example, all types of pure PILL are inductively defined, and if $ is a closed type then is an inductively constructed type However, some models may contain types that are not inductively constructed For example, in syntactical models, any basic open type, such as the type lists is not inductively constructed We define positive and negative occurences of free type variables in inductively defined types as usual The type variable occurs positive in the type and the positive occurences of a type variable in are the positive occurences of in and the negative in The negative occurences of in are the positive in and the negative in The positive and negative occurences of $ in are the positive and negative occurences in for The rest of the type constructors preserve positive and negative occurences of type variables & If is an inductively defined type in which the free type variable appears only negatively and the appears only positively, then we can consider as a functor by defining the term $ & " free type variable which behaves as the morphism part of a functor, ie, it respects composition and preserves identities We define by structural induction on This construction immediately generalizes to types with less or more than two free type variables, all of which appear only positively or negatively 7

8 For the (nontrivial) base case of the induction, if, define In the case & we define the term $ & & by % % ) For bang types, we define: 6 & let be 6 in For tensor types, we define: 3 & let 6 0 & be 3 in 6) The last case is the case of polymorphic types: $ 3 $ 3 & Lemma 30 The term respects composition and preserves identities, ie, for,,, and ', + 0 3 ) 3 < " -/ - -/ Proof The proof proceeds by induction over the structure of, and most of it is the same as in [], except the case of tensor-types and These cases are essentially proved in [] Notice that in the proof of Lemma 30 we do not need parametricity Suppose We shall write for < The type of is Notice that we apply to, since is of intuitionistic function type ( instead of ) By the previous lemma, defines a bifunctor First we consider this in the case of only one argument: Lemma 3 (Graph lemma) Assuming identity extension, for any type with occuring only positively and any map ' Likewise, suppose ) is a type with only occuring negatively Then identity extension implies 3 where is +6 "" 76 8

Proof We will only prove the first half of the lemma; the other half is proved the same way Since occurs only positively in, we will assume for readability that $ has type & By parametricity of, for any pair of admissible relations & &' and Let 3 be arbitrary If we instantiate () with and, we get " using the identity extension schema Since -/ -/ ", () and using we get -/ " ie, 6 76 6 We have thus proved implies To prove the other direction, instantiate () with the admissible relations, for 3 Since -, 6 So for any and we have 6 implies 37 Initial algebras - implies 6 This just means that Suppose is an inductively constructed type in which occurs only positively As we have just seen, such a type induces a functor for each We aim to define an initial algebra for this type Define the closed type $ and define as and as fold $ fold & in in 3 fold 3 9

Lemma 3 For any algebra, fold is a map of algebras from " in to 3, ie, the diagram commutes Proof For 6 as desired in fold fold fold in 6 in 6 fold 6" In words we have shown that in defines a weakly initial algebra for the functor defined by in for each Notice that parametricity was not needed in this proof Lemma 33 Suppose 8 and 8 % 8 extensionality, Proof Since % are algebras for, and is a map of algebras, ie, % % Then, assuming identity extension and % fold is a map of algebras so by the Graph Lemma (3) and by Lemma 3 Clearly fold fold so for any 6 ie, as desired % % % % % % " fold eq$, and thus, by identity extension,, fold fold $ % fold 6 % fold fold Lemma 3 Using identity extension and extensionality, fold in & Proof By Lemma 33 we know that for any type, fold fold in The left hand side of this equation becomes fold and, since the right hand side is simply the lemma follows from Lemma 3 in 30 6" fold - and fold in

8 8 Theorem 3 Suppose & is an algebra and % of algebras from in to Then if we assume identity extension and extensionality, % % Proof By Lemma 33 we have fold in Lemma 3 finishes the job We have shown that in defines an initial algebra 38 Final Coalgebras fold is a map fold As in section 37 we will assume that is a type in which occurs only positively, and this time we construct final coalgebras for the induced functor Given any we can define the closed type $ $ with combinator defined as pack $ pack 6 $ 6 Define $ $ with combinators defined by where unfold $ " out unfold ) 6 pack 6 out 6 76 $ ), let 3 be in unfold let be in 3 Lemma 36 For any coalgebra, the map unfold is a map of coalgebras from to out Proof We need to prove that the following diagram commutes unfold unfold out But this is done by a simple computation out unfold 6& out pack 6 pack,6 ) 6 unfold 6 3

Lemma 36 shows that out is a weakly final coalgebra for the functor induced by on each Notice that parametricity was not needed here for Lemma 37 Suppose %,,, is a map of coalgebras If we assume identity extension, then the diagram unfold commutes internally Proof Using the Graph Lemma, the notion of % unfold being a map of coalgebras can be expressed as % & % Now, by parametricity of unfold, unfold % eq unfold which is exactly what we wanted to prove Lemma 38 Using extensionality and identity extension, unfold & out is internally equal to the identity on Proof Set % unfold out in the following By Lemma 36 % is a map of coalgebras from out to out, so by Lemma 37, % % Intuitively, all we need to prove now is that % is surjective Consider any $ 0 For any coalgebra map ' ), we must have, by Lemmas 3, 3, and 9, #6 6" so by identity extension and parametricity of, 6, 6 ) +6 Using this on the coalgebra map unfold from to out we obtain 6 ) 6 out, unfold 6 By Lemma 3 this implies that, "76 6 out, unfold 60 which implies "3, 0 3 let 6 be 3 in, out unfold 6 3

8 8 using Lemma 36 In other words, if we define where $ 0 ", to be let 6 be in ) out unfold 60 then Now, suppose we are given $ and terms such that ) Then, by (3) and parametricity of " (3) from which we conclude $ & (Here we use % for the inverse relation of % ) Using parametricity, this implies that, for any 6 we have 6 +6 Thus, since was arbitrary, we may apply the above to and get 6 +6 & let 3 be +6 in out, unfold 3 If we write '6 let 3 +6 be in unfold 3 then, since is a closed term, so is, and from the above calculations we conclude that we have $ 0 76 out ) 6 Now, finally, % 6 & unfold out 6 & pack out 6 $ 0 out 6 $ 76 60 where we have used extensionality Thus is a right inverse to %, and we conclude % 6 % 6 % 6 & 6, Theorem 39 Suppose is a coalgebra and % a map of algebras from to out Then if we assume identity extension and extensionality % unfold 33 is

Proof Consider a map of coalgebras into out: out By Lemmas 37 and 38, unfold unfold & out Theorem 39 shows that out is a final coalgebra for the endofunctor on each induced by for 39 Recursive type equations In this section we consider inductively constructed types and construct closed types such that In Sections 37 and 38 we solved the problem in the special case of occuring only positively in, by finding initial algebras and final coalgebras for the functor induced by The first observation we use is that we may split the occurences of in in positive and negative occurences So our standard assumption in this section is that we are given a type &, in which occurs only negatively and only positively, and we look for a type, such that 3 This section details the sketch of [0] 39 Parametrized initial algebras Set to types $ Lemma 30 Assuming identity extension and extensionality, for, to internal equality) the unique % commutes internally such that & Now, induces a contravariant functor from types in in is (up Proof One may define in as a polymorphic term in $ by in 3 '6 76 fold "3 3

By parametricity we have in " which, by the Graph Lemma (Lemma 3), means that in in in which in turn amounts to internal commutativity of the diagram of the lemma Uniqueness is by initiality of in (in, proved as before) used on the diagram & in in 39 Dialgebras Definition 3 A dialgebra for is a quadruple 3 & such that and are types, and and 3 are morphisms A morphism of dialgebras from to is a pair of morphisms %, % ', such that Lemma 3 If % % is a map of dialgebras and % % are isomorphisms, then % % is an isomorphism of dialgebras Proof The only thing to prove here is that % *, % *, & is in fact a map of dialgebras, which is trivial Remark 33 If we for the type consider the endofunctor & defined by " &, then dialgebras for are exactly the algebras for dialgebras are maps of algebras for and initial dialgebras correspond to initial algebras, maps of Theorem 3 Assuming identity extension and extensionality, initial dialgebras exist for all functors induced by types, up to internal equality Proof In this proof, commutativity of diagrams will mean commutativity up to internal equality Set & & Then, defines a contravariant functor Define " " & Since is defined as the final coalgebra for a functor, we have a morphism ' out " & 3 " 3

and since is defined to be an initial algebra, we get a morphism in & We will show that 3 in out is an initial dialgebra Suppose we are given a dialgebra 3, such that 3 Since in is an initial algebra, there exists a unique map 3 in 3 3 and thus, since out is a final coalgebra, we find a map % making the diagram out " " % We claim that % % defines a map of dialgebras The second diagram of commute Set % Definition 3 is simply () The first diagram of 3 follows from the commutativity of the composite diagram 3 3 where the top diagram commutes by Lemma 30 3 in 3 in 3 3 3 Finally, we will prove that % % is the unique dialgebra morphism Suppose we are given a map of dialgebras 3 from 3 ; ( to By the first diagram of Definition 3, we have a commutative diagram in 3 Since clearly () also commutes when is substituted for %, by (strong) initiality of in, we conclude that Finally, by the second diagram of Definition 3 we have commutativity of out So since out is a final coalgebra we conclude % " " () () 36

393 Compactness Theorem 3 (Compactness) Assuming identity extension and extensionality, for all types in which *, *, occurs*, only positively, *, in is internally a final coalgebra and out is internally an initial algebra Furthermore in and out can be written as terms of PILL Proof By Theorems 3 and 39 in is an initial algebra, and out is a final coalgebra for Consider % % in % out Since is a fixed-point operator, we know that out in *, commutes Since in is a coalgebra, we also have a map going the other way, and since out is a final coalgebra, % -/ Since in is an initial algebra, we know that % *, -/ *, So in out as coalgebras and out in as algebras, internally Lemma 36 Assume identity extension and extensionality Let 3 in out be the initial dialgebra from the proof of Theorem 3 Then 3 *, *, out in is also an initial dialgebra internally Proof In this proof, commutativity of diagrams is up to internal equality Suppose we are given a dialgebra We will show that there exists a unique morphism of dialgebras from + 3 *, *, out in to 3 By Theorem 3, for all types *,, in is a final coalgebra for the functor *, 3 3 &3, and out is an initial algebra for the functor " Let commute Define % be the unique map making the diagram commute We define % to be in to be the unique map making 3 3 % " out 3 and prove that % % is a map of dialgebras The first diagram of Definition 3 is simply (6) Commutativity of the second diagram follows from commutativity of in in 37 " (6) (7)

where commutativity of the last diagram follows from Lemma 30 Finally, we will show that if ' is another map of dialgebras from 3 out then % and % By the second diagram of Definition 3 we know that 3 3 in 3 3 3 *, *, in to (8) commutes Clearly, if *, we substitute for % in (7), we obtain ' a diagram that commutes by Lemma 30 So, using the fact that in is a final coalgebra on (8), we get The first diagram of Definition 3 implies that 3 " commutes Comparing this to (6) we obtain % out 3 3, by initiality of out Theorem 37 Assuming identity extension and extensionality, for all types where occurs only negatively and only positively, there exists a type and a map 3, such that 3 3 *, is an initial dialgebra up to internal equality *, Proof As usual commutativity of diagrams will be up to internal equality We have a unique map of dialgebras % % 3 in out 3 *, out *, in We claim that % % is also a map of dialgebras from 3 in out to 3 *, out need to prove commutativity of the diagrams in 3 3 out in out 3 3 *, in To prove this we but the fact that % % is a map of dialgebras tells us exactly that 3 in 3 3 out in 3+ out 3 3 " and these two diagram are the same as the above but in opposite order Thus, by uniqueness of maps of dialgebras out of 3 in out, we get % % Since % % is a map between initial dialgebras, % is an isomorphism 38

Now define 3 & to be in % *, - from 3 3 *, to 3 in out, since the diagrams proving -/ 3 in 3+ in Then clearly -/ *, % *, % 3 out in 3 is a morphism of dialgebras to be a map of dialgebras are 3 3 Clearly the first diagram commutes, and the second diagram is just part of the definition of % % being a map of dialgebras Thus *, % - defines an isomorphism of dialgebras from 3 3 *, to 3 in out, as desired Corollary 38 Assuming identity extension and extensionality, for all types, where occurs only negatively and only positively, there exists a type such that 3 in each % *, Proof The isomorphism is in - Notice that the closed terms 3 and 3 & always exist, independent of the assumption of parametricity We use parametricity to prove that they are each others inverses LAPL-structures In this section we introduce the notion of LAPL-structure An LAPL-structure is a model of LAPL First, however, we call to mind what a model of PILL is and how PILL is interpreted in such a model (for a full description of models for PILL and interpretations in these, see eg [7,,,, ]) A model of PILL is a fibred symmetric monoidal adjunction such that is fibred symmetric monoidal closed; the tensor in is a fibred cartesian product; is equivalent to the category of finite products of free coalgebras for the comonad on ; is cartesian; has a generic object and simple products with respect to projections forgetting, where is of the generic object See [] for detailed explanation of this definition PILL is interpreted in such models as " follows A type is interpreted as an object using the SMCC structure to interpret and the comonad to interpret, and we interpret a term 6 8 6 <; as a morphism, : ' in, where Notice that we denote the morphisms in by The comonad structure on induced by the adjunction gives us two natural transformations ) ) and -/ These are defined in the internal language as 6 ) let 6 be in 6 ) let 3 6 be in 3 39

It turns out that we may interpret the intuitionistic part of the calculus, that is, the terms in the calculus with no free linear variables, in For suppose we are given such a term Then the interpretation of this term in ( ( Since $ ( that such a term corresponds to in It is easy to prove that using Lemma 3 of [] 76 8 6 ( 6 8 8 6 ; is 8 ( <; ' ( ( is strong) and, we have, using the adjunction, <; 8 $ ( ; 6 '( * -/ 8 <; " Definition A model of PILL is a model of PILL, which models a fixed point operator Definition A pre-lapl-structure is a schema of categories and functors $ such that the diagram is a model of PILL is a fibration with fibred finite products is an indexed first-order logic fibration [] which has products and coproducts with respect to projections in [], where is applied to the generic object of is a faithful product-preserving map of fibrations 0

a contravariant morphism of fibrations: 3 a family of bijections 3 "$# # for and in and in, which is natural in the domain variable is natural in commutes with reindexing functors; that is, if is a morphism in, then & where is the cartesian lift of Notice that is only defined on vertical morphisms '& ( is a morphism in and % By contravariance of the fibred functor we mean that is contravariant in each fibre Since is uniquely defined by the requirements on the rest of the structure so we will often refer to a pre-lapl structure simply as the diagram in item Strictly speaking, we should denote the bijection by since it depends on all these, but for ease of notation we simply write We now explain how to interpret a subset of LAPL in a pre-lapl structure The subset of LAPL we consider at this stage is LAPL without admissible relations and without the relational interpretation of types We interpret the full contexts of the considered subset of LAPL in the category context : %6 6 " %$ : # : as follows A is interpreted as $ ( ( $*)+ ) ) " where the interpretations of the types is the usual interpretation of types in For notational convenience we shall write <; ' ; for the interpretation of in, that is for 8 <; ' -, (note the subscript ), where, is the projection, in The propositions in the logic are interpreted in as follows ' Let denote the diagonal map, then 6 '3" <6 /0033 "

& / 00 33 where denotes the left adjoint to reindexing along <;& ; To interpret 6 (, recall that a context $ ( where ( is the usual interpretation of types in in 6 We may therefore interpret (, $ ( '( # Likewise, is projection in The existential quantifiers 6 ( Now we can define %6 976 ( 6 '3" <6 is interpreted as and the product refers to the fibrewise product using the right adjoint to reindexing along the projection $ ( ( interpreted using right adjoints to reindexing functors related to the appropriate and are interpreted using left adjoints to the same reindexing functors Quantification, over types and is interpreted using respectively right and left adjoints to,& is the lift of the projection, in $ to To be more precise, one may, where & easily show that for for the interpretation of PILL '( wellformed, and so the cartesian lift of, is a map:, using the corresponding result and we define where $ is the right adjoint to,& $ Definable relations with domain and codomain in contexts into The definable relation $ & # are interpreted as maps from is interpreted as the projection, and +6 " *, 76 " We now define the interpretation of +;, for a definable relation ;" and terms for Next, if ;,, we define &76 ", then <; +60" +;, &76 " of the right types First, +60" where,,, are the projections,, One may think of the isomorphism as a model-theoretic version of Lemma 7

of, ie, a contravari- whose components are To interpret admissible relations, we will assume that we are given a subfunctor ant functor with domain and codomain as and a natural transformation all monomorphic Thus, for all, we can consider as a subobject of We think of as the subset of all admissible relations (since the isomorphism allows us to think of as the set of all definable relations) We may interpret the logic containing admissible relations by interpreting % as Admissible relations are interpreted as maps into For this to make sense we need, of course, to make sure that the admissible relations in the model (namely the relations that factor through the object of admissible relations) in fact contain the relations that are admissible in the logic We need to assume that of the functor Definition 3 A pre-lapl structure together with a subfunctor of is said to model admissible relations, if is closed under the rules of Figure and Rule 8 holds Lemma In the interpretation given above of the subset of LAPL excluding the relational interpretation of types in a pre-lapl structure modeling admissible relations, if is a proposition in the logic, and is a term, then ; 6 where,,, are the projections, 3, 76 3 Proof By induction on the structure of Cases using the fact that 76 ', ;, ; 76 ; 6, and * 76 #3 <; are easy from definitions, simply in the PILL model The cases, etc, are just the fact that the fibrewise structure of is preserved by reindexing, and the cases of the quantifiers is by the Beck-Chevalley condition Lemma For +6 " +; 60 +; and ; ) and 3' Proof This follows from Lemma and naturality of in : Assume for simplicity that and are empty Observe %6 <; 6 +;, where is the unit of the adjunction Now +6 " +; 60 which using Lemma and the calculation above gives +;" *, *, +; 6 " 6 " +6" *, %6 " +60" & +; +; 6, 3

8 # Given a pre-lapl structure modeling admissible relations, we may define a fibration *# which we think of as a model consisting of admissible relations We first define the category by the pullback $ We write an object in over an object objects is as & triples where and are objects in is an admissible relation, ie a vertical map The fibre of *# over over and,, in Here,, are first and second projection respectively out of morphisms A morphism is a pair of morphism +; in and respectively, such that +; where we have left the inclusion of into implicit " respectively and Reindexing with respect to vertical maps in is done by composition Reindexing objects of "$# with respect to lifts of maps in $ $ is done by reindexing in the fibration $ Reindexing of morphisms in "$# with respect to maps in is done by reindexing each map in $ This defines all reindexing since all maps in can be written as a vertical map followed by a cartesian map Remark 6 In the internal language, objects of *# are admissible relations A vertical morphisms in "$# from to is a pair of morphisms,, 3 in such that in the internal language the formula 6 " +6" 60 holds (this follows directly from Lemma )

There exist two canonical maps of fibrations: *# On the base category # map an object map ) to and respectively In words, respectively and to and respectively On the total category they map a relation to its domain and codomain Lemma 7 The fibration *# has products in the base, a generic object and simple products with respect to projections in forgetting the generic object The maps preserve this structure Proof The category has products:,, (see [9, Proposition 9]) The fibration has a generic object -/ -/, since a morphism into this from in consists of pairs of types and a morphism from to We now show that we have products with respect to projections forgetting the generic object Given a relation we can define as 8 $& & & 3 $ &' &' $ " $ $ & +;" & & +; We will to show that this defines a right adjoint to weakening Suppose we have another relation & We will use the usual adjunction in, where a map corresponds to ; 6 +;,6 $ ; preserves relations iff does, but it is clear that ; 8 +; We need to prove that %6 " $& & 6 +; 6 iff 6 " 6 & &' ;,6 which establishes the bijective correspondence between maps, proving that we have in fact defined a product ' ' ' '' ' ' ' & $, with ",

Lemma 8 The fibration *# the two maps are fibred strict symmetric monoidal functors Proof We prove that the constructions, has a fibrewise SMCC-structure and on definable relations given in Section define a fibrewise symmetric monoidal structure on *# Notice that since is closed under the rules of Figure, Proposition 3 tells us that the constructions on definable relations of Section indeed do define operations on *# First we will prove that the two operators do in fact define functors on *# That is, we need to check that if then and, if +;, then +; +; & +; +; ;- ' To see that defines a functor, suppose 6 +; Recall that and let 3 3 +; ; +6 be ;- 6 6 ; be in let let 3 3 be +; ; & and in 3 3 let 3 3 be ;- in PILL Notice then that We need to show that in 3 3 in 3 3 let 3 3 be in +; 3 +; 3 " 3 63 +; 3 +; 3 3'63 3 3 6 The result now follows from the assumption that To prove that is a functor, suppose and +60" Then clearly +; +6", as required We need to show that is right adjoint to, and that the adjoint components are natural in Since we are given a similar adjunction in, all we need to show is that +;" iff ; ; where ;" are the maps corresponding to in the adjunction on Suppose first that +;" 6 and +; The definition of the latter says exactly that, for all, we must have ; 6 Now, suppose ;" and 6& 6 By Lemma 9 +6 6 " and so ; +6 6 " ; +6 6 & ; 6 6 Hence, since We now proceed to prove that the functors "$# (likewise for ), we are done define a fibred SMCC structure on 6

The unit in a fibre is ')+ ' where is the unit in the appropriate fibres of maps giving the isomorphisms, ) ) ", " ) ) The are simply pairs of the corresponding maps in the fibrewise SMC-structure of These maps satisfy the coherence properties simply because the maps in do the same One has to check that the maps defined by pairing maps in fact define maps in *#, ie, that they preserve relations One direction of the isomorphism is given by the map 6 76 To see that this preserves relations, suppose 6& Since 3'*),+ +6<, 0'*),+ by Lemma 9 For the other direction, consider given as 6 6 let 6 6 be in Then the map is simply, and we have proved earlier that it now suffices to prove that preserves relations So suppose 6& 6 0')+ 6 By definition of 0'*),+ we can conclude that 6 6 The isomorphism, is obtained using the adjunction as follows, ' ' ' ' - ' '' ' - ' '' ' & and it is easily seen that this isomorphism is given by pairs of the usual maps in isomorphism comes from By construction, the functors ' '' '% ' '' '% ' '' '% are fibred strict symmetric monoidal functors Likewise the Lemma 9 The fibration "$# has a fibred comonad structure induced map this comonad to the fibred comonad on the nose by the functor The maps Proof We need to check that defines a functor, ie, that if, then It is easy to see that 6" 60 6" since 6 & +6 Now the result follows from Rule 8 The comonad maps are given by & and These preserve relations by Lemma 3, and the commutative diagrams of a fibred comonad are preserved since they hold for the fibred comonad on $ Lemma 0 The fibration *# ) making it a fibred linear fibration The maps 7 has natural transformations " 0 0'*)+ preserve this structure on the nose 7

and Proof The maps are given by, which preserve relations by Lemma 33 The necessary diagrams commute by the same diagrams for the fibred comonad on, and the functors preserve the structure on the nose by construction If we define *# to be the category of finite products of coalgebras [], we obtain a PILL-model *# "$# and two maps of PILL-models This model need not be a PILL -model, since for pre-lapl- does not necessarily preserve relations structures Definition An LAPL-structure is a pre-lapl-structure modeling admissible relations, together with a map of PILL-models from to *# "$# such that when restricting to the fibred linear categories, together with is a reflexive graph, ie, - In the following, we will often confuse with the map of fibred linear categories from to *# We need to show how to interpret the rule &&' " 9 & ' & " in LAPL-structures Since preserves products in the base and generic objects, in context 8 & ' It thus makes sense to define ' & to be context using thus obtaining For is a relation from, so all we need to do now is to reindex this object We reindex it to the right & ' ) &', we define &&' " 3 & 8 " &&', 3 to

where by we mean the pairing % Remark To model a version of Linear Abadi & Plotkin Logic for unary or other arities of parametricity as in Remark, the functor should, have corresponding arity and the domain and codomain of the bijection should be changed accordingly Furthermore instead of considering the fibration of binary relations "$# we should consider a fibration of relations of the appropriate arity Soundness In this section we prove that the interpretation of LAPL in LAPL-structures is sound First, we present a series of reindexing lemmas Lemma 3 If 76 # 3 is a proposition in the logic, and ; is a term, then ; 6 3, <; ", &76 3 where,,, are the projections, Lemma If 76 3, is a definable relation in the logic, and ; is a term, then &76, <; ", & ; 6 Notice that Lemma 3 differs from Lemma since the latter only concerns the interpretation of the part of the logic not including the relational interpretation of types Proof The two lemmas above are proved simultaneously We only include the proof of the former, for which we only need to extend the proof of Lemma to the case of But this follows easily by induction using the latter lemma Lemma If 3 then 76 3, 3 where, is the obvious projection Likewise, if then &76 # where, is the obvious projection Proof The lemma can be proved in a way similar to Lemmas 3 and, 9

Lemma 6 If then - and where the vertical line in -/ - denotes the cartesian lift Proof We know that -/ since the corresponding statement holds in the PILL-model and the functors commute with reindexing Now one proceeds by simultaneous induction on and For and for one uses that commutes with reindexing For one uses the Beck-Chevalley condition, as is also done for the cases of and The remaining cases either follow by induction or from the fact that the fibrewise structure in (, etc) is preserved by reindexing Lemma 7 If then, Likewise, if If then, Proof By simultaneous induction Lemma 8 If 3 is a definable relation and $ 3 then Likewise, If 3 - is a definable relation and $ 3 " $ 3 & then $ 3 -/ The same holds for substitution of admissible relations Proof By simultaneous 3 induction on fibrewise structure in is preserved by reindexing and, using naturality of, Beck-Chevalley and the fact that the 0

Lemma 9 If is a proposition then $ where, is the obvious projection Likewise, if $ &, 0+ 3 is a definable relation then where, is the obvious projection The same holds for substitution of admissible relations Proof Again by simultaneous induction Theorem 0 (Soundness) The interpretation given above of LAPL in LAPL-structures is sound with respect to the Rules and Axioms 9-6 Proof Rules 9-6 hold since the interpretation of quantification is given by adjoints to weakening, considering Lemmas, 7, 9 above Rules -7 hold since substitution corresponds to reindexing as in the lemmas above Rule 8 is proved exactly as in [] Rule 7 holds since externally equal maps are interpreted equally in the model, by soundness of the interpretation of PILL Clearly internal equality is an equivalence relation Rule 8 is required to hold in Definition 3 Rules 9- all hold since preserves SMCC-structure, generic objects, simple products and To prove soundness of Rule, it suffices to prove soundness of but 76 " *, -, &76 " +6 " +6" ) +6 " +6" 76 " +6 " 76 " To prove Axiom 6, notice that is required to be a functor This means that it maps to a morphism from ')+ $ to the relational interpretation of is a reflexive graph, this map must be 3 Since '*),+ and $ we get $ By the requirement, that Completeness Theorem (Completeness) There exists an LAPL-structure with the property that any formula of LAPL over pure PILL holds in this model iff it is provable in LAPL Proof We construct the LAPL-structure syntactically, giving the categories in question the same names as in the diagrams of the definitions of pre-lapl- and LAPL-structures The category has as objects sequences of the form ' % where we identify these contexts up to renaming (in other words, we may think of objects as natural numbers) A morphism from into is a sequence of types 9 such that all ( are well-formed in context &

) 6 8 Objects in the fibre of over are well-formed types in this context Morphisms in this fibre from ; to are equivalence classes of terms such that 8 6 ;, where we identify terms up to external equality Equivalently, we may think of morphisms as terms ; Composition is by substitution, and reindexing with respect to morphisms in is by substitution Objects in the fibre of over are well-formed sequences of types in this context Morphism in this fibre from 9 to : +; ( are equivalence classes of sequences of terms ( :, such that for each ( the term 6 8 <; ( ' ( +; ( is well-formed, where the sequences +; ( ;7( and are identified if, for each (, is externally equal to ; ( Reindexing with respect to morphisms in is by substitution 8 6 ; 6 The functor maps a morphism to 8 ; The functor going the other way maps a sequence of objects ( ( to ( +; ( It maps a morphism from ( to ( to 8 ( (, (6 ( ( ( let be in let 6 be ( ; ( in For further details of the term model for PILL see [] The category has as objects in the fibre over well-formed contexts of LAPL: vertical morphism from to " %$ # "% &&' " %% : ' : : is a triple, consisting of a morphism of definable relations, and a sequence of admissible relations : ( ( ( and & ( represented by the same type morphism and the definable relations 9 and and admissible relations : and :, respectively, if, for each ( ($ ( ) and are provable in the logic, where, as usual, ( ( is short for 6 ( " ( ( +6 ( " ( ( +6 ( " ( " and likewise for ) The fibre of the category over a context ) # A in the sense of morphisms in, a sequence 9, such that ( ( We identify two such morphisms, the formulas The inclusion functor is the obvious one Reindexing is by substitution has as objects formulas in that context, where we identify two formulas if they are provably equivalent These are ordered by the implication in the logic Reindexing is done by substitution, that is, reindexing with respect to lifts of morphisms from is done by substitution in type-variables, whereas reindexing with respect to vertical maps in is by substitution in term variables and relation variables An easy fibred version of the completeness proof in [] shows that together with the functors described above form a PILL model The fibration clearly has fibred products formed by appending contexts, and the inclusion functor is clearly faithful and product-preserving We need to prove that is an indexed first-order logic fibration with products and coproducts with respect to simple projections in The fibrewise bicartesian structure is given by Fibred simple products and coproducts are given by quantifying over relations and variables, simple products in the composite fibration is given by quantifying over types We can in fact prove that the composite fibration has all indexed products and coproducts (in particular, that it has equality)

; ; % Suppose ; 6 represents a morphism from to consist of both definable and admissible relations, and the vector is a concatenation of the corresponding vectors of admissible and definable relations from the definition above) We can then define the product functor in as: We define coproduct as: % % $ 6 60 & 6 6 60 % 6 60 & 6 6 60 % 6 60 We remark that the equality we will actually be using in the model is the obvious and and The functor 76 of item is defined as +; & 76 " 6 "# +6 " $ (the vectors % The required isomorphism is just the isomorphism given by Lemma 7 The functor is defined as &&' +; &' & +6 " $ We have defined a pre-lapl-structure modeling admissible relations If we construct as in the definition of LAPL-structure, we obtain: Objects &&' " " # " Morphisms A morphism from to % consists of two morphism in : and " " ) " " % # +; 6 " " +; 6 a morphism from to in, and a sequence of admissible relations and a sequence of definable relations such that, for all i,j, &&' " " # " ( &&' ( " ( ' " ) ) ) ) " " 3 & " As in these morphisms are identified up to provable equivalence of the definable relations 3

8 The fibre of *# over an object in becomes: Objects Equivalence classes of definable relations " " " # " " & " # & is a pair of mor- Morphisms A morphism ; from ' " to &' " phisms 3, such that it is provable in the logic that: 6 ' +60" +; 60 We will construct the map as follows On the base categories We define as a map of fibred linear categories from *# is defined on objects as " 3 " & 9 8 & " 9$ on the objects of the total categories (and on the morphisms of the base category) as &&' & " & To define +; on 7; morphisms of the fibre categories, suppose We define To see that in fact is a map from to, notice that the Logical Relations Lemma (33) tells us that +; 7; which means exactly that 7; $ 7;" ; $ to +; +; 7; Rules 9- tell us that is a strict fibred symmetric monoidal closed functor preserving products and on the nose Since the and of the fibred comonad on *# are simply and it is clear that preserve these as well Now, by definition, a formula holds in this LAPL-structure iff it is provable LAPL Parametric LAPL-structures Definition A parametric LAPL-structure is an LAPL-structure with very strong equality in which identity extension hold in the internal logic Recall that very strong equality implies extensionality We ask that identity extension and extensionality hold because this means that all the results from Section 3 apply to the internal logic of the LAPL-structure Strong equality will be used to conclude that properties proved in the internal logic also hold externally, as exemplified in the following subsection, where we show how to solve recursive domain equations in parametric LAPL-structures

: : Solving recursive domain equations in parametric LAPL-structures Suppose we are given a fibred functor where by we mean taking the opposite category in each fibre As in [] we can easily prove that is isomorphic to a functor whose object part is given by some type, since on that form Definition An endofunctor transformation ; has as generic object in $ So in the following we shall assume that is 3, for an SMCC, is called strong if there exists a natural 3 preserving identity and composition: ) + 3 3 ; The natural transformation is called the strength of the functor (Note that we here used exponential notation for the closed structure in ) One should note that ; in the definition above represents the morphism part of the functor in the sense that it makes the diagram commute, for any pair of morphisms < This follows from the commutative diagram Definition 3 A strong fibred functor is a fibred functor

; on a fibred SMCC, for which there exists a fibred natural transformation from the fibred functor * * to " * ; The natural transformation is called the strength of the functor Definition We will say that the fibred functor satisfying commutativity of the two diagrams of Definition in each fibre $ assumed to be defined on objects as for some strong if there exists a closed term ; $ & such that the family +; as above, is polymorphically (each fibre to the th power) is a strength of the functor in the sense of Definition 3 The term ; is called the polymorphic strength of For example, the interpretation of any inductively constructed type induces a polymorphically strong fibred functor as described in Section 36 Theorem In a parametric LAPL-structure, for any polymorphically strong fibred functor there exists 3 in for the terminal object of a closed type such that Proof The proof of the earlier sections give us this result in the internal language, since we may use identity extension and extensionality in the internal language The functorial interpretation of types of Section 36 should be substituted by the polymorphic strength of the functor Since internal equality implies external equality, we get the result in the real world Remark 6 Since the functor is fibred, we may reindex to get a family of objects such that for each, 3 in the fibre, where is the unique map in $ satisfies Remark 7 Parametric LAPL-structures do not in general model recursive types, that is, we do not have for all types 6 Concrete Models a type such that In this section we describe a parametric LAPL structure based on admissible pers over a universal domain as advocated by Plotkin [0] Pers are known to model the typed -calculus, and admissible pers further facilitates a fixed point operator As noted in Section, to model PILL one has to provide a fibred symmetric monoidal adjunction We do this by constructing a regular symmetric monoidal adjunction and then define the fibration pointwise The standard example is a lifting functor and a forgetful functor This is also the case here albeit slightly obfuscated, as lifting is coded in the language of the universal domain This is an adaptation of [0] Finally, to be able to model polymorphism, the entire construction is done fibred Parametricity is then ensured by a yet-to-be-described completion process, but first we present the clean version: Let be a reflexive cpo, ie a pointed -chain-complete partial order such that we have 6

( ( both Scott-continuous and satisfying where denotes the cpo of continuous functions from to We assume, without loss of generality, that both and are strict It is standard that there exists strict continuous functions,, and such that for all :, We use to denote ( Notice that and, Definition 6 An admissible partial equivalence relation on is a partial equivalence relation satisfying strict, on +6 -chain complete For and -chains in : 6 6 Definition 6 For and % pers on, define the set of equivariant functions from to % as % 6 +6 % % and the set of strict equivariant functions from to as %# % Note % % Definition 63 For and % pers on, define on % or % ' % the equivalence relation We write for the category of partial equivalence relations over Recall that it has partial equivalence relations over as objects and that a morphism % is an equivalence class in % ' Elements of are called realizers for Definition 6 We define the category of admissible partial equivalence relations over as the full subcategory of on the admissible pers Lemma 6 There is a faithful functor mapping an admissible per to the set of equivalence classes and an equivalence class of realizers to the map of equivalence classes they induce Proof This is well-defined since to realizers are equivalent precisely when they define the same map of equivalence classes Theorem 66 The category is a sub-cartesian closed category of ' by 7

' ' ( Proof We recall the constructions It is straightforward to verify that the resulting pers are admissible The terminal object is the admissible per defined by The binary product of and % is % This is an exhaustive description, understood that only pairs are related in the product The exponential of and %, %, is given by of admissible pers and strict continuous functions is the full-on- with morphisms & % ' Definition 67 The category objects subcategory of Note that in Theorem 68 % % ' equivalence classes in %, morphisms are required to have a strict continuous realizer is a cartesian sub-category of Proof Obvious using that,,,, and Theorem 69 The category Proof The tensor of and % is are strict is symmetric monoidal closed % % % % % % This complicated looking definition is most easily understood through the functor : The equivalence classes of the tensor product are those of the product with the modification that all pairs where one of the coordinates are related to has been gathered in one big equivalence class It can thus be seen as a quotient of the product The unit of the tensor is defined by This definition is not taken out of the blue is actually in the image of a lifting functor to be defined later Notice the if construct on, which will be available on all lifted relations:, &, &, &, & 8

( ' ( Thus for,, can be read as if then else We will use this to construct realizers The exponential of and %, %, is given by % % The proof consist of a series of straightforward verifications % % For later use we shall mention how regular subobjects look in this category We use that is a regular subobject of, if is an admissible per to express Lemma 60 Proof In there is a standard way of constructing an equalizer out of a subset of the equivalence classes This also works here, and the image of an equalizer is easily seen to be admissible Thus all regular subobjects have a representative, which is tracked by the identity on We also need to know the following fact about admissible pers Lemma 6 If is an arbitrary set, and for all ( is an admissible per over (, ( is an admissible per over then Proof We intersect relations, which may be seen as sets of pairs Thus we have the following equation ( ( ( ( which makes the statement obvious, as all ( are admissible 6 The connection to CUPERs In [] Amadio and Curien show how complete uniform pers over a universal domain allows on to solve domain equations on the per level As we claim, the same is true for admissible pers, a comparison is natural There are, however, some technical issues which makes this a little difficult Cupers are defined over a universal solution to the domain equation In the category ( that is then the colimit of the Denoting Defining by of pointed directed complete partial orders and injection-projection pairs It is known -chain we have by the cocone property fo each ) ( the diagram: we can define a cuper on as a relation such that 9

( If If and then for all, ( then So apart from using directed-complete rather than chain-complete cpos and living on a universal domain solving a slightly different domain equation, cupers live on a domain with a known structure, and this structure appears in their definition Thus the only reasonable way to compare the two notions is to consider a suitably adapted notion of admissible pers over the above described We then find, that the cupers form a proper subset of the admissible pers It is noticeable, that cupers facilitate an ordering of the equivalence classes and thus allows one to solve recursive domain equations, while admissible pers achieve this by modeling polymorphic -calculus and calling upon parametricity Hence the two approaches are somewhat different 6 Lifting We now define a notion of lifting, to establish an adjunction between and Our notion of lifting is essentially the one in [0], specialized to the partial combinatory algebra defined by, and for Let denote the power set of Define the map And then the map &, & Notice the if construct available on a liftet relation: If Thus, by, by is an admissible per then,, &,,, then else, where of course represents for any admissible per % We also have a lift and an unlift : If then and if then, This is convenient for constructing realizers Similarly define, for admissible pers and % %, the map " % by which reads if ' And then the map can be read if ' &, ( then lift( (unlift )) else by +;, for % A tedious, but straightforward, verification shows that the definitions of all make sense, and that forgetful functor Theorem 6 There is a monoidal adjunction and defines a functor There is an obvious,, And calling upon parametricity is, as far as we know, only possible after the deployment of a parametric completion process 60

; Proof One first shows that is left adjoint to by ' ' For % in is given by the realizer, all tracked by in the ordinary sense The unit of the adjunction is given, the required unique % % in ;, +;, ", such that % ' ; where is a realizer for To show that the adjunction is monoidal it suffices by to show that the left adjoint is a strong symmetric ' monoidal functor (see [] for an explanation) To this end, we must exhibit an isomorphism ' and a natural isomorphism % % This is mostly straightforward; we just include ' the definition of, % % : it is the morphism tracked by the realizer which reads,,,,,,, if else else then if, then ",, lift of unlift,,), unlift Following a similar chain of thought, the inverse is tracked by,,, ",,, which reads then lift of, unlift, lift of,) unlift else if 6

( ( 63 Going fibred In order to model polymorphism, we do a fibred version of the adjunction presented in the last subsection, thus arriving at the PILL-model * Define the contravariant functor objects: ( ( where for all ( ( ( ( ( % ( ( morphisms: ( * by mapping set to the category with, ( ( % (, ( is an object of, where, for all ( and For a function ', the reindexing functor is simply given by composition with Define the contravariant functor given by mapping set to the category with objects: ( ( where for all (, ( is an object of ( ( ( ( % ( ( morphisms: where for all (, ( ( % ( and ( For a function ', the reindexing functor is again simply given by composition with and The functors and That we have two contravariant functors is obvious The Grothendieck construction then gives us two split fibrations, easily lift to fibred functors between these two fibrations (we abuse notation and also denote the fibred functors by and ' ) Explicitly, on objects ( ( ' ( ( and on vertical morphisms ( ( ' ( ( Likewise for These are not recursive definitions, they simply look so because of the reuse of letters Theorem 63 and are split fibred functors and Proof It is obvious that and are split fibred functors; the second part follows immediately from Theorem 6 6 A domain-theoretic model of PILL is a split fibred strong monoidal adjunction To show that (9) is a model of PILL it remains to be shown that has a generic object and simple products Lemma 6 The set is a split generic object of the fibration The fibration has simple split -products satisfying the Beck-Chevalley condition Proof The first part is obvious For the second part, one uses the usual definition for uniform families of ordinary pers and verifies that it restricts to admissible pers: We recall from [9] that given any projection in, the right adjoint to, is given on objects by intersection:, # & By lemma 6 the resulting per is admissible (9) 6

Theorem 6 The diagram (9) constitutes a model of PILL Proof Given the preceding results it only remains to verify that () the structure in the diagram models the polymorphic fixed point combinator and that () is equivalent to the category of products of free coalgebras of For (), the required follows, as expected, because the pers are strict and complete In more detail, the reasoning is as follows: It is well-known that there is a Scott-continuous function giving fixed-points through iterated application at Since realizers are Scott-continuous functions in, every realizer has a fixed-point in given by If for some admissible per, $, then, since is strict, respects, and is chain-complete, Thus the equivalence class of exists and is a fixed-point of the morphism represented by This is applicable both in and, but is not so interesting on strict morphisms It is, however, in that we model the calculus, and thus here we want a fixed-point combinator albeit only for some morphisms, namely those of type, corresponding to morphisms of Intuitively, we wish to take such a morphism, transpose it, grab the fixed-point in and call the whole process a morphism in This is possible, since transposition cascades to the level of realizers The function that transposes a morphism and returns the fixed-point of the result is continuous The fixed-point function is in $, for any, and thus its code is a member of Precomposing with a uniform realizer for before taking the code, one easily obtains the polymorphic fixed-point combinator 0 Writing this out, one arrives at,, For (), observe that by [, Proposition ] applied to Theorem 68 it suffices to show that is equivalent to the cokleisli category of the adjunction, but this follows from the fact that is a forgetful functor 6 A parametric domain-theoretic model of PILL In this section, we introduce a parametric version of the thus far constructed model It is essentially obtained through a parametric completion process such as the one described in [] for internal models (as mentioned in the Introduction, we will generalize that completion process to produce parametric LAPLstructures in []) We will arrive at the diagram * * Our construction is based on reflexive graphs and since our strategy is to obtain relational parametricity for admissible relations (to also model the fixed point combinator in the parametric model), we consider the set of diagrams % where is a regular subobject of % in (It is crucial that subobject is in the category with strict maps it means that will relate the equivalence class of in to the equivalence class of in % Identifying with, we define the base category by (0) 63

Objects: objects are natural numbers Morphisms: is an -tuple, % :, where each ( is a pair ( ( ( is a map of objects ( ( is a map, that to two vectors of objects of ( ' satisfying ( satisfying associates maps of subobjects ) ) % ) ( ( ( ' & ' where the regular subobjects are to be calculated in We now describe * and As objects they basically contain an (indexed) per and an (indexed) relational interpretation of this per As morphisms they have uniformly tracked morphisms that respect admissible relations We wish to model admissible relations as regular subobjects in, so we introduce the notation for We plan to use the Grothendieck construction, and so define indexed categories: is defined with Objects: is a morphism in from to Morphisms: is a uniformly tracked family of morphisms ' ' such that ' of % That is uniformly tracked means that there is a strict continuous function ; such that ( Furthermore this should respect relations: % % Quite similarly Objects: is a morphism in ' ; "7; is defined as the category with from some to ' ; ' % ; Morphisms: is a uniformly tracked family of morphisms ' ' such that ' where is the forgetful functor That we now ask for morphisms of removes the demand, that the uniform tracker be strict Again this should respect relations: % % % ; Here is still a regular subobject in ; "7; Note that the only difference between the two definitions is the choice of category in which the ' required to be morphisms Definition 66 Define " * on 6 "7; "7; of are

" " objects by where and & % & % morphisms by Define Lemma 67 If % Lemma 68 " are both functors, and ", then * % * in a similar way using instead of and Proof Easy given lemma 67 and the fact that for all admissible pers, ' & ' ensures that the realizer ; for the unit of required universal property By an easy extension of Theorem 66, we have: Theorem 69 is fibred cartesian closed also defines a natural transformation ( Lemma 67 " with the Proof It turns out to be easy, since the product of two regular subobjects turns out to be a regular subobject of the product, and the exponent of two regular subobjects turns out to be a regular subobject of the exponent Since the adjunction works on the level of realizers and realizers are uniform, the adjunction holds Theorem 60 is fibred cartesian and fibred symmetric monoidal closed Proof We just present the SMCC structure: The tensor product of 3 and in the fibre is denoted by, where and % %) % tracked by and defined by ) & " &, is defined as the image of the map % % % % ;,,,, ",,,, which on pairs of pairs have the following behavior: The unit " " of the tensor is given by the object ' % 6

The exponential of 3 and in, is & defined by & where and % is defined by % & % " % " % This is an exhaustive description, in the sense that only pairs are ever related To verify the adjunction, we use that we know that it holds in the first component and then check that the bijection can be restricted to realizers that define morphisms in the second component; the latter is a direct consequence of the way the relational interpretations of and are defined Lemma 6 " is a fibred symmetric monoidal adjunction Proof This proceeds much as in the unfibred case We show that " is a fibred strong symmetric monoidal functor We must provide a morphism and a natural transformation, but we can simply use the same realizers as before, since everything has been defined coordinatewise and these realizers are independent of the specific pers, and hence are uniform realizers Lemma 6 is a split generic object of Proof Obvious Lemma 63 If is an object of 0, then, where %$ +6" ' $ $ +6" % % 6" ' % ' 6" and % is an object of Lemma 6 * 60" % 9 % 6 " % 60" 6 " +676 % " has simple -products, we must define a right adjoint to, This is done by extending the construction of the previous lemma in an obvious way to a functor Proof The construction is as in [9, Section 8] Given a projection, Proposition 6 The diagram (0) constitutes a PILL model Proof It only remains to verify that the structure models the fixed point combinator Here we simply use the from Theorem 6, which works since relations are strict and chain complete 66

We now proceed to show that this new PILL need just two more fibrations, fibre of * over has as Objects maps model can be extended to an LAPL-structure For this we and The Morphisms ; is a family +; ' ' ) and reindexing is given by composition The fibre of is a preorder with Objects maps, such that over an object Morphisms There is a morphism if Here reindexing is with respect to morphisms in is given by composition, whereas reindexing with respect to morphisms in * is given by inverse image Lemma 66 simple -products and -coproducts is a fibration with fibred products, and % is an indexed first-order logic fibration with Proof is a first-order logic fibration with generic object and all simple products and coproducts By Lemma A8 in [] we can construct the pullback * obtaining that is a composable fibration with the desired qualities Yet this is not quite the right fibration Fortunately we have 67

: by the isomorphism mapping to And now * is a pullback The bottom half is a pullback by definition, the map And the top one easily is a pullback as well As have the desired qualities We can then define the functor operates as follows preserves products, the leftmost composable fibration Lemma 67 is a faithful and product-preserving map of fibrations to be the fibred version of It is now time to define the contravariant map of fibrations This is defined at index on Objects by &, where denotes powerset, Morphisms by + +6" +6" Lemma 68 Proof is a contravariant map of fibrations can be equivalently defined as # which makes the statement clear We can then define a family of bijections and * * such that for all # # * by % & % 68

Lemma 69 is a bijection, which is natural in the domain variable, is natural in,, and which commutes with reindexing functors We have now proved: Proposition 630 The diagram * () constitutes a pre-lapl structure Now we define a subfunctor of on Objects by &, One can now show that is closed under all the constructions performable on admissible relations and that it contains all graph relations Lemma 63 The structure in diagram () and model admissible relations Proof We refer to figure and provide only a part of a formula to hint at which construction we are debating: : Equality on a type is modeled as the diagonal subobject of admissible relation because it is isomorphic to by the continuous functions +; 6 +; This corresponds to an and, : Reindexing an admissible relation by a strict continuous function (ie ) is bound to give +;" an admissible relation We consider chain-completeness: Given two index-wise related chains *, +; in, taken on these gives us two index-wise related chains in Since is chain- is continuous the limits of the original chains is complete their limits are related in +;, and since in the inverse image of the limit in +60" +6" : Conjunction is modeled by intersection, under which admissible relations by lemma 6 are stable 6 +6" 6 : In our model we are allowed to reason classically and thus rephrase the formula to +60" We recognize this as with the strip " added This is easily admissible +6 " 0 76 : This is all classes except those where exactly one component is Thus we have all the lifted classes and then We recognize this as the lift of truth +6 3" 76 : swapping the abscissa and ordinal axis does not break admissibility 6 : Again we reason classically: Either holds or it does not If it holds the formula equals, if it does not we get all the classes where 6 or This is admissible since no chain can converge out of : This is all classes This is admissible 69

# : +6" : If does not hold we get all classes If does hold we get which is admissible Quantifications: All quantifications are modeled through intersections and are thus taken care of by lemma 6 Having come so far, we move on to describe *# and from Section Recall that is defined as the pullback which means that has as Objects triples where Morphisms triples in and : is an indexed family of maps ' % " % ', assigns a set to a vector of admissible pers where and ) where and are evaluated on the combined lists of admissible pers ) In this concrete case *# can be described as follows: Given an object, the fibre of *# over has as Objects triples such that and are objects of is an indexed family of maps ' % % ' ) Morphisms A morphism is a pair of morphisms +; in and 6" 3 60" Note that we now have two obvious projections Finally we can define the required functor : % ; +6 " and, respectively, such that 3 % 3 ; +6" *# are morphisms over over and respectively and ) For the base categories, is defined on 70

Objects by $ ( Morphisms by $ ( ( and for the total categories, Objects by % Morphisms by ( % ( ' is defined on This definition on morphisms is legal because preserves relations In order to show that preserves tensor products, we need the following lemma Lemma 63 The tensor product in *# ) & where is calculated pointwise, ie for 3 ' 3 3 0 3 Proof We argue that this construction defines a left adjoint to holds, on the level of realizers even, which is not hard to show Lemma 633 is a map of linear -fibrations % can be described as The standard curry-uncurry-adjunction Proof We must show that preserves $ The constructions in the two categories are virtually identical except for until application of lemma 63 To check the case of we consider the logical expression for &&' : +6 " 6 6 60 6 equates 6 Hence 60 6 The expression express the fact that no lifted class is related to in Further since provides us with unlifted versions of its argument, 6 states that liftet classes are related in only if their unlifted versions are related in This is an exact description of the lifting performed by the functor It is easy to see that -/ and -/ Theorem 63 The diagram in () constitutes a parametric LAPL-structure Proof By the preceding results it is clear that it is an LAPL-structure; it only remains to show that it is a parametric such Extensionality holds since the logic is essentially given by regular subobjects, which means that we have very strong equality [9], and thus also extensionality The parametricity schema is easily verified to hold Example 63 To ease notation in this example we shall write +60" subobjects % +6 We consider the type iff for all % pers and all regular subobjects The domain of for 6" 60" for regular, as we do in LAPL We will also leave <6 implicit, and simply write for $ By definition %, and +6" 6 6 6 +6 6 76 contains the elements 7 and, in particular

Lemma 636 Suppose Then Proof Consider the two functions given by, where is the code of the identity function, and being the 6 first projection Both are continuous and since -/ the sequence of elements This sequence is strictly increasing Now, if then so so Further, 6 6 : *' 6 : 6 : 6 : is injective Define Lemma 637 The per is a admissible Proof Direct consequence of the lemma above Proposition 638 Suppose Then either or Proof Consider the discrete admissible per : Then given 76 consider the regular subobject given by +6 so is admissible, simply because it contains no interesting increasing chains Clearly succ succ 6 ie, if succ, then <6 for all 76 and so, and if succ 6 +6, for all 76, so As we have seen, there are no other possibilities for succ Proposition 639 Suppose, then Proof Analyzing the above proof we see that succ By considering the regular subobject given by for some, then, we conclude succ succ Acknowledgments We gratefully acknowledge discussions with Milly Maietti, Gordon Plotkin, John Reynolds, Pino Rosolini and Alex Simpson 7

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