VoiceXML Programmer s Guide

Transcription

1 VoiceXML Programmer s Guide VOICEXML PROGRAMMER S GUIDE 1

2 BeVocal, Inc. 685 Clyde Avenue Mountain View, CA Part No Copyright BeVocal, Inc. All rights reserved. 2 VOICEXML PROGRAMMER S GUIDE

3 Table of Contents Preface Audience Conventions How to Use This Guide References Getting Started VoiceXML Environment Tags and Elements Simple Example Documents Applications Dialogs Properties Grammars Events Links Universal Commands and Grammars Procedural Logic User Interaction Flow of Execution Explicit Transition Recognition-Triggered Transition Subdialogs Collecting Input and Playing Prompts Forms Form Items Form-Item Variables Execution of a Form User Interaction VOICEXML PROGRAMMER S GUIDE 3

4 3. Event Handling Predefined Events Default Event Handlers Application-Defined Event Handlers Events in Subdialogs Throwing Events Application-Defined Events Fetching and Caching Resources How Fetching and Caching Work Requests and Responses Using Multiple Caches Fundamentals of Controlling Caching What You Can Control from VoiceXML Prefetching Resources Prefetch Cache Details Fetch Hints Restrictions on Prefetching Handling Fetching Delays Timeouts Background Audio Queued Prompts when Fetching Controlling the Use of Cached Resources Maximum Age Maximum Stale Time Caching Mimicking Response Headers Submitting Complex JavaScript Objects Using Multiple-Recognition Multiple Recognition Features N-Best Recognition Multiple Interpretations Combining the Features Working with Multiple Recognition Using N-Best Recognition Enabling N-Best Recognition Checking for Multiple Utterances VOICEXML PROGRAMMER S GUIDE

5 Selecting an Utterance Example Using Multiple Interpretations Enabling Multiple Interpretations Checking for Multiple Interpretations Selecting an Interpretation Example Using Both Features Together Enabling Both Features Checking for Multiple Results Selecting a Result Simple Example Generating a Subdialog Controlling Outbound Calls Call Status Limitations on Outbound Calls Interactions Without an Outbound Call Interactions During a Transfer Without a Transfer Grammar With Transfer Grammars Interactions During a Dialed Call Putting a Dialed Call on Hold Listening to a Dialed Call With Listen Grammars Without a Listen Grammar Interrupting a Dialed Call VoIP and Outbound Calls Go-Back Facility Retracting User Responses Go-Back Stack Stack Entries Go-Back Destinations Menus Mixed-Initiative Forms Input Items Enabling the Go-Back Facility VOICEXML PROGRAMMER S GUIDE 5

6 Activating the Universal Grammar Setting the Minimum Stack Size Controlling Go-Back Behavior Suppressing Retraction Customizing Go-Back Using the Go-Back Facility Selecting the Minimum the Stack Size Using Blocks Using Subdialogs Using Variables TTS and Recorded Voice Selection Specifying TTS Voices Supported TTS Voices Property syntax Property Property Example Errors Voice tag Syntax Voice tag description Specifying Recorded Voices Supported Recorded Voices Example Lists of Fallback Voices Rationale Algorithm for Selecting the Voice Selecting Voice Example Best Practices for Voice Selection Overriding Recorded Voices Dynamic SSML Introduction Using Dynamic SSML Examples and Notes Errors SSML Document Extensions to the SSML spec VOICEXML PROGRAMMER S GUIDE

7 10. SOAP Client Facility Locating and Identifying SOAP Services Calling SOAP Methods Type conversion SOAP Headers SOAP Methods are JavaScript objects Error Handling For WSDL-Based Services Error Handling for Non-WSDL-Based Services Tags Tag Summary Tag Index Tag s <assign> <audio> <bevocal:connect> <bevocal:dial> <bevocal:disconnect> <bevocal:enroll> <bevocal:foreach> <bevocal:hold> <bevocal:listen> <bevocal:register> <bevocal:verify> <bevocal:whisper> <block> <break> <catch> <choice> <clear> <data> <disconnect> <div> <dtmf> <else> <elseif> <emp> <emphasis> VOICEXML PROGRAMMER S GUIDE 7

8 <enumerate> <error> <example> <exit> <field> <filled> <foreach> <form> <goto> <grammar> <help> <if> <initial> <item> <lexicon> <link> <log> <mark> <menu> <meta> <metadata> <noinput> <nomatch> <object> <one-of> <option> <p> <paragraph> <param> <phoneme> <prompt> <property> <pros> <prosody> <record> <reprompt> <rethrow> VOICEXML PROGRAMMER S GUIDE

9 <return> <rule> <ruleref> <s> <say-as> <sayas> <script> <send> <sentence> <speak> <sub> <subdialog> <submit> <tag> <throw> <token> <transfer> <value> <var> <voice> <vxml> Properties Property Summary Property Index Property s audiofetchhint audiomaxage audiomaxstale bargein bargeintype bevocal.audio.capture bevocal.audio.outputvolume bevocal.dtmf.flushbuffer bevocal.fetchaudio.allfetches bevocal.fetchaudio.extend bevocal.fetchaudio.flushqueue bevocal.fetchaudio.sounds VOICEXML PROGRAMMER S GUIDE 9

10 bevocal.finaltimeout bevocal.goback bevocal.grammar.interpretationtype bevocal.grammar.phoneticpruning bevocal.grammar.weightfactor bevocal.grammar.wordtransitionpenalty bevocal.hotwordmax bevocal.hotwordmin bevocal.incrementerroronnsp bevocal.locale bevocal.logging bevocal.maxdialogerrors bevocal.maxerrors bevocal.maxinterpretations bevocal.mingoback bevocal.securelogging.enabled bevocal.securelogging.key bevocal.security.key bevocal.sounds.listening bevocal.sounds.maskrecognitionlatency bevocal.sounds.recognition bevocal.transfer.terminatetones bevocal.utterance.prefix bevocal.voice.name bevocal.vxml.maxrecognitionlatency caching completetimeout confidencelevel datafetchhint datamaxage datamaxstale documentfetchhint documentmaxage documentmaxstale fetchaudio fetchaudiodelay fetchaudiominimum VOICEXML PROGRAMMER S GUIDE

11 fetchtimeout grammarfetchhint grammarmaxage grammarmaxstale incompletetimeout inputmodes interdigittimeout maxnbest maxspeechtimeout recordutterance recordutterancetype scriptfetchhint scriptmaxage scriptmaxstale ssmlfetchhint ssmlmaxage ssmlmaxstale sensitivity speedvsaccuracy termchar termtimeout timeout universals Variables Variable Summary Variable Index Variable s _event _message application.lastaudio$ application.lastresult$ session.bevocal.timeincall session.bevocal.version session.iidigits session.telephone.ani session.telephone.dnis VOICEXML PROGRAMMER S GUIDE 11

12 14. JavaScript Functions and Objects JavaScript Constants bevocal.outboundrequestid bevocal.sessionid _addheader bevocal.cookies.addclientcookie bevocal.cookies.deleteclientcookie bevocal.cookies.getclientcookie bevocal.cookies.getclientcookies bevocal.enroll.removeenrolledphrase bevocal.getproperty bevocal.getversion bevocal.log bevocal.soap.servicefromwsdl bevocal.soap.servicefromendpoint bevocal.soap.locateservice bevocal.soap.soapexception bevocal.soap.soapfault bevocal.soap.faultdetails VOICEXML PROGRAMMER S GUIDE

13 Preface VoiceXML is a markup language for writing telephone-based speech applications. This document describes BeVocal VoiceXML, which is compliant with the W3C VoiceXML Version 2.0 Specification. Audience This document is for software developers using the BeVocal Café development environment. It assumes you are familiar with the basic concepts of HTML. Conventions Italic font is used for: Introducing terms that will be used throughout the document Emphasis Bold font is used for headings. Fixed width font is used for: Code examples Tags and attributes Values or text that must be typed as shown Filenames and pathnames Italic fixed width font is used for: Variables Prototypes or templates; what you actually type will be similar in format, but not the exact same characters as shown How to Use This Guide Part I of this guide explains how to use VoiceXML features. A new application developer typically reads these chapters completely and in order. Chapter 1, Getting Started introduces VoiceXML and its major features. Chapter 2, Forms describes VoiceXML forms. Chapter 3, Event Handling describes events that can be thrown during the execution of a VoiceXML application and how events are handled. Chapter 4, Fetching and Caching Resources explains how an application can control the way VoiceXML documents and other resources are fetched and cached.

14 PREFACE Chapter 5, Using Multiple-Recognition describes how the BeVocal VoiceXML interpreter can provide multiple recognition results. Part II of this guide explains how to use Extended VoiceXML features. A new application developer typically reads those chapters which are relevant for his application. Chapter 6, Controlling Outbound Calls describes the BeVocal VoiceXML call-control features, an extension to VoiceXML. Chapter 7, Go-Back Facility describes the BeVocal VoiceXML go-back facility, an experimental extension to VoiceXML. Chapter 8, TTS and Recorded Voice Selection describes the BeVocal VoiceXML TTS and Recorded Voice Selection facility, an experimental extension to VoiceXML Chapter 9, Dynamic SSML describes the BeVocal VoiceXML Dynamic SSML facility, an experimental extension to VoiceXML Chapter 10, SOAP Client Facility describes the BeVocal VoiceXML SOAP Client facility, an experimental extension to VoiceXML Part III of this guide provides reference descriptions of the various components of the VoiceXML language. Application developers typically do not read these chapters from start to finish, but instead use them to look up information about the various tags, properties, and so on. Chapter 11, Tags describes the tags that make up VoiceXML. Chapter 12, Properties describes the properties that can be set to control the behavior of a VoiceXML application. Chapter 13, Variables describes predefined variables that are available in VoiceXML applications. Chapter 14, JavaScript Functions and Objects describes predefined JavaScript functions that are available in VoiceXML applications. References For additional or related information, you can refer to: VoiceXML Version 2.0 Specification. VoiceXML Forum. ( VoiceXML Tag Summary. BeVocal. ( Grammar Reference. BeVocal. ( JavaScript Quick Reference. BeVocal. ( 2 VOICEXML PROGRAMMER S GUIDE

15 PART 1 Using VoiceXML This part explains how to use VoiceXML features: Chapter 1, Getting Started Chapter 2, Forms Chapter 3, Event Handling Chapter 4, Fetching and Caching Resources Chapter 5, Using Multiple-Recognition

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17 1 Getting Started VoiceXML is a markup language derived from XML for writing telephone-based speech applications. Users call applications by telephone. They listen to spoken instructions and questions instead of viewing a screen display; they provide input using the spoken word and the touchtone keypad instead of entering information with a keyboard or mouse. This chapter describes: VoiceXML User Interaction Flow of Execution Collecting Input and Playing Prompts VoiceXML Just as a web browser renders HTML documents visually, a VoiceXML interpreter renders VoiceXML documents audibly. You can think of the VoiceXML interpreter as a telephone-based voice browser. As with HTML documents, VoiceXML documents have web URIs and can be located on any web server. Yet a standard web browser runs locally on your machine, whereas the VoiceXML interpreter is run remotely at the VoiceXML hosting site, for example. And you use your telephone to access the VoiceXML interpreter. Environment In order to support a telephone interface, the VoiceXML interpreter runs within an execution environment that includes a telephony component, a text-to-speech (TTS) speech-synthesis component, and a speech-recognition component. The VoiceXML interpreter transparently interacts with these infrastructure components as needed. For example: Text strings in output elements are rendered using TTS. Connection issues (picking up the incoming call, detecting a hang-up, transferring a call) are handled by the telephony component. Listening to spoken input from the user and identifying its meaning is handled by the speech-recognition component. Tags and Elements VoiceXML uses markup tags and plain text. A tag is a keyword enclosed by the angle bracket characters (< and >). A tag may have attributes inside the angle brackets. Each attribute consists of a name and a value, separated by an equal sign (=) and the value must be enclosed in quotes.

18 GETTING STARTED Tags occur in pairs; corresponding to the start tag <keyword> is the end tag </keyword>. Between the start and end tag, other tags and text may appear. Everything from the start tag to the end tag, is called an element. For example, the following three lines constitute a prompt element: <prompt> What is your telephone number? </prompt> If there are no other tags or text between the start and end tag, a syntactic shorthand is permitted. You can precede the closing angle bracket ( > ) of the start tag with a slash ( / ) and omit the end tag. For example, instead of writing a value element as: <value expr="result"></value> you can use the shorthand notation: <value expr="result"/> Because the syntax specifies the end of each element, the VoiceXML interpreter can check that the entire document has been received. If one element contains another, the containing element is called the parent element of the contained element. The contained element is called a child element of its containing element. The parent element may also be called a container. Although both HTML and VoiceXML use markup tags, the two languages use tags differently. Whereas the markup tags in HTML describe how to render the data, the markup tags in XML (and consequently in VoiceXML) describe the data itself. This allows an XML interpreter or browser to display the data in whatever way is appropriate. BeVocal VoiceXML generally complies with the VoiceXML 2.0 Specification. It also includes several handy extensions that you can use if you choose. VoiceXML Tag Summary lists any differences between BeVocal VoiceXML and the standard. Tip: VoiceXML conforms to XML standards; the formats for VoiceXML tags are more strictly defined than are the formats in HTML. If you are used to HTML and not XML, remember that all container elements require end tags and all attribute values must be in quotes. Simple Example In VoiceXML, the <form> element is analogous to an HTML form that contains items for the user to enter. In VoiceXML forms, each logical piece of information to be collected from the user is identified with a <field> tag. The form in the following example collects one piece of information from the user. Once this information is obtained, execution proceeds to the field s <filled> element. Other tags used in the example include the following: The <script> tag specifies a block of client-side JavaScript code. The <var> tag declares a variable to be used within the form. The <prompt> tag produces audio output for the user. The <assign> tag assigns a value to a variable. The <value> tag evaluates an expression and produces spoken output of the result. This example requests a number from the caller, computes the factorial of that number, and repeats the answer to the caller. <?xml version="1.0"?> <!DOCTYPE vxml PUBLIC "-//BeVocal Inc//VoiceXML 2.0//EN" 6 VOICEXML PROGRAMMER S GUIDE

19 VoiceXML " <vxml version="2.0" xmlns=" <!-- This piece of JavaScript code actually computes the factorial. --> <script> <![CDATA[ function factorial(n) {return (n <= 1)? 1 : n * factorial(n-1);} ]]> </script> <!-- Primary VoiceXML form for the application --> <form id="computefactorial"> <!-- variable to hold result --> <var name="result"/> <!-- The field element holds 1 piece of info gotten from the caller. --> <field name="num" type="number"> <!-- Ask the caller for the information. --> <prompt>please say a number </prompt> <!-- The filled tag is run when the interpreter --> <!-- gets a valid answer from the caller --> <filled> <!-- Compute the factorial and assign it to the result variable --> <assign name="result" expr="factorial(num)" /> <!-- Tell the caller the answer --> <prompt>the factorial of <value expr="num"/> is <value expr="result"/> </prompt> <!-- Close everything off. --> </filled> </field> </form> </vxml> VoiceXML contains no explicit instructions about how to present the prompt, please say a number or how to present the results. In theory, these could be presented textually on a different kind of browser. In practice, the example document is run as a telephone application and results in conversations such as the following. Application: Please say a number. User: 4 Application: The factorial of 4 is 12. VOICEXML PROGRAMMER S GUIDE 7

20 GETTING STARTED Documents An executable VoiceXML file is called a document. The VoiceXML interpreter loads a document file to execute it. Every VoiceXML document must start with header information that conforms to the XML standard: <?xml version="1.0"?> <!DOCTYPE vxml PUBLIC "-//W3C/DTD VoiceXML 2.0//EN" " <vxml version="2.0" xmlns=" These headers describe the language in which the document is written: The first tag indicates that the document is an XML document. This tag is required. Always use this tag exactly as specified; it must be the very first characters in the document. To be a legal XML document, the first 4 characters of any XML file (including a VoiceXML document) must be: <?xm No characters, not even whitespace characters such as space or newline, can come before these 4 characters in a VoiceXML document. The second tag identifies the Document Type Definition (DTD), which is used to validate that the contents represent well-formed VoiceXML. This tag is optional. A DTD describes the format of the data that might appear in an XML document. That is, the DTD defines the valid tags by specifying what attributes each tag can have and what child tags or other content each tag can contain. If your document contains only standard VoiceXML elements, you can use the DTD shown above. If you use any of the BeVocal VoiceXML extensions to VoiceXML, you ll need to use the correct DTD. In this case, you replace the DOCTYPE element with the following: <!DOCTYPE vxml PUBLIC "-//BeVocal Inc//VoiceXML 2.0//EN" " You should include a DOCTYPE declaration during development, as it allows better error checking by the interpreter. You may remove it during deployment for performance. The third tag identifies the version of VoiceXML used in this document and the designated namespace for VoiceXML. This tag is required. For VoiceXML 2.0, this tag should always include these 2 attributes. It can also include optional attributes described in the section on the <vxml> tag. Apart from headers and possibly comments, all the content in a VoiceXML document is contained within a <vxml> element, that is, between the <vxml> start tag and the </vxml> end tag. Applications A VoiceXML application consists of one or more documents. Any multidocument application has a single application root document. Each document in an application identifies the application root document with the application attribute of the <vxml> tag: <vxml version="2.0" xmlns=" application="myapproot.vxml" > Whenever the interpreter executes a document, it loads that document. If the document specifies an application root document, that document is also loaded. 8 VOICEXML PROGRAMMER S GUIDE

21 VoiceXML You can use an application root document for global items or interactions that you want to be active throughout the application. For example, suppose the application root document myapproot.vxml declares a variable named company that has an initial value of BeVocal: <vxml version="2.0" xmlns=" <var name="company" expr="bevocal">... This variable has application scope. That is, any document in the application can use the variable. Dialogs Within a document, a user interacts with dialogs, in which the application produces auditory output, typically asking for information. The user provides input by speaking or pressing keys on the telephone. User speech must be recognized and its meaning interpreted. The telephone key input is interpreted as a sequence of tones in the Dual Tone Multifrequency (DTMF) signalling system. VoiceXML has two kinds of dialogs: forms and menus. A form interacts with the user to fill in a number of fields. Every field has an associated variable, called its input-item variable, or just input variable. Initially, the variable has a value of undefined. It is filled in when the speech-recognition engine recognizes a valid response in a user utterance. Note: In VoiceXML 1.0, an input-item variable was known as a field-item variable. A menu presents the user with a number of choices; it transitions to a different dialog based on the user s selection. Forms The VoiceXML <form> tag defines a form and the <field> tag defines a field in a form. You specify the name of the input variable with the name attribute of the <field> tag. You can use the input variable s name in expressions to refer to the stored value. In the example in Simple Example on page 6, the input variable is named num: <field name="num" type="number"> When the user says the number, the number is stored in the num variable. Then the interpreter proceeds to execute the field s <filled> element. Here, the num variable in the <assign> element is evaluated before being passed as the parameter to the factorial function. <assign name="result" expr="factorial(num)" /> Menus The <menu> tag defines a menu; each choice consists of a <choice> element. The next attribute of a <choice> element specifies the destination dialog to which the interpreter should transition when the user selects that choice. If a <form> or <menu> element is to be the destination of a transition, the id attribute for the destination dialog should specify a unique identifier. For example, the following menu consists of three choices. <menu> <prompt> Please choose one of <enumerate/> </prompt> <choice next="#movieform"> local movies </choice> <choice next="localbroadcast.vxml#radioform"> VOICEXML PROGRAMMER S GUIDE 9

22 GETTING STARTED local radio stations </choice> <choice next=" national TV listings </choice> </menu> The prompt in this menu includes an <enumerate> tag. This tag lets you set up a template for an automatically generated description of the choices. By default, the <enumerate> template simply lists all the choices. In the above example, the prompt is Please choose one of local movies, local radio stations, national TV listings. The destination dialog specified by the next attribute can be in the current document or in a different document: If the user says local movies, the interpreter transitions to the dialog named MovieForm in the same document. If the user says local radio stations, the interpreter transitions to the dialog named RadioForm in the document localbroadcast.vxml. If the user says national TV listings, the interpreter transitions to the first dialog in the document tv.vxml in the national TV web site. Properties You can set properties to customize the behavior of the interpreter. The <property> tag specifies the property to set and the value for that property. Various properties control how the interpreter behaves when prompting the user for input, recognizing speech or DTMF input, and fetching documents and other resources. For additional information, see Chapter 12, Properties. Grammars The speech-recognition engine uses grammars to interpret user input. See the Grammar Reference for details on creating and using grammars. Here, we only cover a portion of the relevant information. Each field in a form can have a grammar that specifies the valid user responses for that field. An entire form can have a grammar that specifies how to fill multiple input variables from a single user utterance. Each choice in a menu has a grammar that specifies the user input that can select the choice. A VoiceXML application can use built-in grammars and application-defined grammars. 10 VOICEXML PROGRAMMER S GUIDE

23 VoiceXML Built-in Grammars The following basic grammars are built into all standard VoiceXML interpreters: Grammar Type boolean currency date digits number phone time Recognizes a positive or negative response. Recognizes an amounts of money, in dollars. Recognizes a calendar date. Recognizes a sequence of digits. Recognizes a number. Recognizes a telephone number adhering to the North American Dialing Plan (with no extension). Recognizes a clock time. BeVocal VoiceXML contains additional built-in grammars as an extension to standard VoiceXML: Grammer Type airport airline equity citystate stockindex street streetaddress Recognizes an airport name or code, such as DFW or Dallas-Fort Worth. Recognizeds an airline name or code, such as AA or American Airlines. Recognizes a company symbol or full name, such as IBM or Cisco Systems. Recognizes US city and state names, for example, Sunnyvale, California. Recognizes the names of the major US stock indexes, such as Nasdaq. Recognizes a street name (with or without street number). Recognizes a street name and a street number. You can reference a built-in grammar in either of two ways: You can use a standard built-in grammar as the type attribute of a <field> element. The example in Simple Example on page 6 uses the built-in number grammar: <field name="num" type="number"> This means that the speech-recognition engine tries to interpret what the user says as a number. You can use any built-in grammar (standard or BeVocal VoiceXML extension) in a <grammar> element by specifying the src attribute with a URI of the form: builtin:grammar/typename For example: <grammar src="builtin:grammar/boolean"/> Application-Defined Grammars Although the built-in grammars can be useful, you typically need to define your own grammars. An application-defined grammar can be specified in the following forms: Augmented BNF (ABNF) form of the W3C Speech Recognition Grammar Format XML form of the W3C Speech Recognition Grammar Format Nuance Grammar Specification Language (GSL) Java Speech Grammar Format (JSGF) VOICEXML PROGRAMMER S GUIDE 11

24 GETTING STARTED A simple grammar can be defined in the document. An inline grammar is defined within the <grammar> element itself. For example, the following inline ABNF grammar matches the words add and subtract. <field name="operator"> <grammar> #ABNF 1.0; root $op; $op = add subtract; </grammar>... With this grammar, if the user says add, the input variable operator is set to add. More complex grammars can be written externally. An external grammar is defined in a file separate from the VoiceXML document file and is referenced by the src attribute of the <grammar> element. For example, the following field uses a grammar rule named Colors in an external XML grammar defined in the file partgrammar.grxml. <field name="part"> <grammar src=" The named rule (Colors in the preceding example) is the one the interpreter will use to start recognition. The specified file may include other grammar rules, which may be used as subrules of the this rule. The grammar for a menu choice can be specified explicitly with a <grammar> child of the <choice> element. Alternatively, a grammar can be generated automatically from the choice text. If the accept attribute of the <menu> tag is set to approximate, the user can say a subset of the words in the choice text to select that choice. Adding this attribute to the preceding example allows the user to say TV listings or just TV to select the third choice: <menu accept="approximate">... <choice...> national TV listings </choice> </menu> Note that the words must be spoken in the correct order; listings, TV would not be recognized. If you want some choices to be matched exactly and others to allow a subset of the words, you can specify the accept attribute on individual <choice> elements. Active Grammars The speech-recognition engine uses active grammars to interpret user input. A field grammar is active whenever the interpreter is executing that field. A menu-choice grammar is active whenever the interpreter is executing the containing menu. A form grammar is active whenever the interpreter is executing the containing form. A form grammar or the collection of choice grammars in a menu can optionally be made active at higher scopes: A grammar with document scope is active whenever the interpreter is executing any dialog in the document. A grammar with application scope is active whenever the interpreter is executing any document in the application. 12 VOICEXML PROGRAMMER S GUIDE

25 VoiceXML If the interpreter is executing one dialog and the user s input matches an active grammar for a different dialog, control transfers to the latter dialog. If the grammar is in application scope, control might transfer to a dialog in a different document. Note that within a field, you can temporarily turn off grammars from higher scopes by setting the field s modal attribute to true. Events The VoiceXML interpreter can throw a number of predefined events based on errors, telephone disconnects, or user input. For example: A no-input event is thrown if the user does not respond to a question. A no-match event is thrown when the user does not respond intelligibly that is, when the user s utterance does not match any active grammar. A help event is thrown when the user requests help. An error event is thrown when any kind of error occurs. An application can define additional events and can use a <throw> element to throw an event of a specified kind. An application can catch an event and take the appropriate response in an event handler. A <catch> element is a general-purpose event handler; its event attribute specifies the kinds of event that it handles. Additional event-handling tags are syntactic shorthand: <noinput>, <nomatch>, <help>, and <error>. Each of these shorthand tags catches one type of event, indicated by its name. For example, a <nomatch> element catches no-match events. When an event is thrown, the associated event handler, if it exists, is invoked. If the handler did not cause the application to terminate, execution resumes in the element that was being executed when the event was thrown. For more information, see Chapter 3, Event Handling. Links A link specifies a grammar that is independent of any particular dialog. A <link> element defines a link. Each <link> element contains a <grammar> element. A link s grammar is active in the scope of the element that contains the link. For example, if the link is in a form, its grammar is active when the interpreter is executing that form. If a link is under a <vxml> element, its grammar has document scope; if the link is in the application root document, its grammar has application scope. Links in a <vxml> element can implement global behaviors. A link can specify one of two possible actions to take if the speech-recognition engine detects a match its grammar: The link can cause a transition to a different location; in that case, its next attribute specifies the destination of the transition. Links, like menu choices, can cause transitions to other dialogs or documents. The link can throw an event; in that case, its expr attribute specifies the event to throw. After the event is handled execution resumes with the element that was being executed when the link grammar was matched. VOICEXML PROGRAMMER S GUIDE 13