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Left: Right-hand-side expressions Up: Syntax Right: Formal syntax of DATR


 DATR sentences represent the statements which make up a description. As we have already seen, there are two basic statement types, extensional and definitional, and these correspond directly to simple extensional and definitional sentences, which are made up from the components introduced in the preceding section.

Simple extensional sentences take the form

    Node:Path = Ext
where Node is a node, Path is a simple path, and Ext is a simple value. Extensional sentences derivable from the examples given in Section 2 include:
    Do:<mor past participle> = done.
    Mow:<mor past tense sing one> = mow ed.
    Love:<mor present tense sing three> = love s.
Simple definitional sentences take the form
    Node:Path == Def.
where Node and Path are as above and Def is an arbitrary descriptor sequence. Definitional sentences already seen in Section 2 include:
    Do:<mor past> == did.
    VERB:<mor form> == "<mor "<syn form>">".
    EN_VERB:<mor past participle> == "<mor root>" en.
Each of these sentences corresponds directly to a DATR statement. However we extend the notion of a sentence to include an abbreviatory convention for sets of statements relating to a single node. The following single sentence:

        Path1 == Def1
        Path2 == Def2
        PathN == DefN.
abbreviates (and is entirely equivalent to):

    Node:Path1 == Def1.
    Node:Path2 == Def2.
    Node:PathN == DefN.
Extensional statements may be similarly abbreviated, and the examples used throughout this document make extensive use of this convention. Such compound sentences correspond to a number of individual (and entirely independent) DATR statements. In this connection, it is worth reiterating that DATR descriptions correspond to sets of statements: the order of sentences, or of definitions within a compound sentence is immaterial to the relationships described.

Finally, there is a pragmatic distinction between definitional and extensional sentences akin to that drawn between the language used to define a database and that used to query it. DATR interpreters conventionally treat all extensional sentences as ``goal'' statements, and evaluate them as soon as they are encountered. Thus, it is not usually possible, in practice, to combine definitional and extensional sentences within a theory. It is not obvious that one would ever wish to do this anyway, but the possibility is explicitly left open in the original definitions of E&G 1989a. Such a statement would respect global inheritance but not local inheritance and could conceivably be useful to achieve some exotic effect.


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Left: Right-hand-side expressions Up: Syntax Right: Formal syntax of DATR
Copyright © Roger Evans, Gerald Gazdar & Bill Keller, Tuesday 10 November 1998