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VoiceXML Developer Series: A Tour Through VoiceXML, Part VI

  • October 6, 2002
  • By Jonathan Eisenzopf
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Grammar rules may contain other grammar rules

A grammar can be made up of other grammar rules, which allows us to create complex grammars by building larger grammar rules that are based on different rule subsets. For example, a grammar that matches a pizza order that's contained in a external grammar is below:

1  #JSGF V1.0
2  grammar phone;
3  <order> = <pizzaSize> 
<pizzaType> [pizza] [with] <topping>+;
4  <pizzaSize> = small | medium | large;
5  <pizzaType> = [hand] (tossed | stretched | thrown) 
6                | [deep] (dish | chicago)
7                | stuffed [crust];
8  <topping> = [and] pepperoni
9                | [and] olives
10               | [and] green peppers
11               | [and] mushrooms
12               | [and] pineapple
13               | [and] anchovies;

The grammar above contains 4 grammar rules. The <order> rule is composed of several other rules that exist in the same JSGF file. These are <pizzaStyle>, <pizzaType>, and <topping>. A sample utterance that would match this grammar is listed below:

I would like to order a small deep dish pizza with olives, 
and pepperoni, and anchovies.

So a JSGF grammar can contain multiple words and phrases represented by subgrammars that, when combined, can match a complex utterance. The grammar above also contains an operator we haven't talked about yet. The last part of the <order> grammar rule uses the <topping> subgrammar followed by a + character. In GSL, we used the same character to match one or more toppings, though it was placed before the subgrammar. Placing a + character after a word, phrase, grouping, or subgrammar, tells the speech recognition engine to look for one or more occurrences of the grammar. In this case, we need to match the list of toppings that the customer would like on their pizza.

Below is a simple VoiceXML fragment that uses the external JSGF grammar listed above.

1  <?xml version="1.0"?>
2  <vxml version="1.0">
3    <form id="pizzaOrder">
4      <block>Hello, thank you for calling Joe's 
Pizza palace. May I take your order?</block>
5      <field name="order">
6        <grammar src="pizzaOrder.jsgf" 
type="application/x-jsgf" />
7       </field>
8     </form>
9   </vxml>


The JSGF format is, in my personal opinion, easier to work with for developing simple grammars. However, I prefer GSL, because it is more widely supported and contains more features for building large and complex grammars. That's not to say that you can't do the same with JSGF. IBM's platform is more than capable of performing the same interactions as a platform from Nuance or Speechworks. In the next

About Jonathan Eisenzopf

Jonathan is a member of the Ferrum Group, LLC based in Reston, Virginia that specializes in Voice Web consulting and training. He has also written articles for other online and print publications including WebReference.com and WDVL.com. Feel free to send an email to eisen@ferrumgroup.com regarding questions or comments about the VoiceXML Developer series, or for more information about training and consulting services.

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