Programming Fundamentals. Lesson 20 Sections

Transcription

1 Programming Fundamentals Lesson 20 Sections

2 Today, we will Study sections, in Haskell. Sections are another class of expressions that represent functions. A section is a binary operation where one of the operands is missing. It represents the unary function in which the missing operand is the argument, so to speak For example, the section (>=10) represents the function (\x -> x >= 10) 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 2

3 Section examples Let s try some sections at the console: Main> (++ " do algarve") "universidade" "universidade do algarve" Main> (++ " do algarve") "orquestra" "orquestra do algarve" Main> ("grande " ++) "carro" "grande carro" Main> ("grande " ++) "festa" "grande festa" Main> (`mod` 100) Main> (100 `mod`) 32 4 Main> (100 `mod`) Main> (`min` 20) Main> (`min` 20) Main> (/2) Main> (2/) Main> (+1) Main> (<= 20) 20.3 False Main> (*10) Main> (10^) Main> (^10) Main> (1/) Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 3

4 Section examples Sections, as lambda expressions, are typically used as arguments to higher order functions: Hugs> map (+1) [5,8,2] [6,9,3] Hugs> map (*2) [5,8,2] [10,16,4] Hugs> map (`min` 20) [19.1, 20.4, 21.4, 16.9, 17.7] [19.1,20.0,20.0,16.9,17.7] Hugs> map (++ ", algarve") ["faro", "tavira", "silves"] ["faro, algarve","tavira, algarve","silves, algarve"] Hugs> filter (<=10) [5,8,12,5,23,44,7,9] [5,8,5,7,9] Hugs> filter (/= 0) [0,3,0,1,1,2] [3,1,1,2] Hugs> map (2^) [0..10] [1,2,4,8,16,32,64,128,256,512,1024] Hugs> map (1/) (map (2^) [0..10]) [1.0,0.5,0.25,0.125,0.0625, , , , , , ] Hugs> until (<0.001) (/2) Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 4

5 Quicksort with sections Using sections, we can further simplify quicksort: qsort :: Ord a => [a] -> [a] qsort [] = [] qsort (x : xs) = qsort (filter (<= x) xs) ++ [x] ++ qsort (filter (> x) xs) This is the standard version of quicksort in Haskell. Remember it, when you have to program quicksort in other languages, in the future. 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 5

6 More exemplary functions The most significant digit: front :: Int -> Int front = until (<10) (`div` 10) The last element: lastelem :: [a] -> a This function is defined by a function expression. Main> front Main> lastelem [1..512] 512 lastelem = head. until (null. tail) tail 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 6

7 Functions with multiple arguments Indeed, a function signature such as f :: a -> b -> c must be read as f :: a -> (b -> c). Thus, f :: a -> b -> c really means that f is a function with an argument of type a and a result of type (b -> c). But (b -> c) is the type of functions with an argument of type b and result of type c. This implies that, in this case, an expression f x, where x is an expression of type a, represents a function from b to c. We conclude that f is a function with one argument and a function result, and not a function with two arguments. 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 7

8 Examples of function calls with missing arguments Note that the missing argument must be the last. Function calls with missing arguments are expressions that represent functions, and can be used as arguments to higher order functions, just like sections: Hugs> filter (elem 'a') ["faro", "aveiro", "porto"] ["faro","aveiro"] Main> map (take 2) ["faro", "aveiro", "porto"] ["fa","av","po"] Main> map (div 100) (map (^2) [1..10]) [100,25,11,6,4,2,2,1,1,1] map (max 10) [6,9,12,3,45,8] [10,10,12,10,45,10] Main> map (zip [ ]) ["lisboa", "porto"] [[(1,'l'),(2,'i'),(3,'s'),(4,'b'),(5,'o'),(6,'a')],[ (1,'p'),(2,'o'),(3,'r'),(4,'t'),(5,'o')]] 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 8

9 IsVowel A function to check whether a char is a owercase vowel: isvowel :: Char -> Bool -- isvowel x = elem x "aeiou" -- isvowel x = flip elem "aeiou" x -- isvowel x = ((flip elem) "aeiou") x isvowel = flip elem "aeiou" Main> isvowel 's' False Main> isvowel 'o' True Main> filter isvowel "portalegre" "oaee" Main> filter (not. isvowel) "portalegre" "prtlgr" Main> filter (not. flip elem "aeiou") "portalegre" 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 9 "prtlgr" flip' :: (a -> b -> c) -> (b -> a -> c) flip' f x y = f y x

10 Exercises Program a function to remove all vowels from a string. Consider the following two functions: f :: Integral a => (a, Int) -> (a, Int) f (x, y) = (div x 10, y+1) d :: Integral a => a -> Int d x = snd (until ((==0).fst) f (x, 0)) What does function d compute? Program a function compute the length of a list, based on the idea embodied by functions d and f above. 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 10

11 Control How many arguments does a Haskell function have? What is a section? What does flip (/) mean? What does a function call with a missing last argument represent? 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 11

12 Tomorrow We will study folds, a group of important higher order functions. 23-Jan-11 Programming Fundamentals - 20 Pedro Guerreiro 12