Compiling Grammar-based Speech Application ... - CiteSeerX

8 Introduction
abstract Pizza = {
cat Input; Order; Number; Size; Topping; [Topping] {1};
fun order : Order → Input;
pizza : Number → Size → [Topping] → Order;
one, two : Number;
small, large : Size;
cheese, ham : Topping;
cat Output;
fun price : Order → Int → Output;
}
Figure 5. Pizza.gf: A GF abstract syntax module.
The abstract syntax declares what can be said in the language. The concrete
syntax describes how to say it. This is done by associating a concrete
representation with each construct in the abstract syntax. In the simplest case,
this concrete representation is a record containing a single string field. Records,
tables and enumerations can be used to implement more complex representations,
for example with gender agreement between nouns and adjectives. The
process of generating a concrete syntax term from a term in the abstract syntax
is called linearization.
One of GF’s strong points is multilinguality. The division of grammar into
abstract and concrete syntax means that it is easy to have multiple concrete syntaxes
for one abstract syntax. This makes it possible to implement multilingual
grammars. In order to avoid re-implementing the domain-independent linguistic
details of a language for each new application grammar, the GF resource grammar
library has been created. It implements the morphological and syntactic
details of a number of languages, and presents a language-independent API to
the application grammar writer. This significantly reduces the effort involved
in translating grammars (Perera and Ranta 2007).
3.1 An Example
As an example of a GF grammar for an interactive speech application, Figure 5
and Figure 6 show the abstract and English concrete syntax for a small pizza
ordering dialogue system. The Input category contains user input, such as “two
large pizzas with ham and cheese”, which corresponds to the abstract syntax
term order (pizza two large [ham, cheese ]). The Output category describes
system output, for example “two large pizzas with ham and cheese cost 7 euros”,
for the abstract syntax term price (pizza two large [ham, cheese ]) 7.
This concrete syntax module uses the new Resource Grammar API with
overloading (Ranta 2006). Each function mkX constructs terms in the resource
grammar category X. For example, the linearization category of the
Order category is the resource grammar category NP. This means that an order