Talk:Definite clause grammar

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Alternate meanings of DCG[edit]

I want to add a page about "Discounted Cumulative Gain", a popular measure for evaluating Web search and related applications, related to information retrieval in computer science.

I would like to make the current DCG page a disambiguation page. Please advise. —Preceding unsigned comment added by Drmadskills (talk • contribs) 17:56, 30 September 2008 (UTC)[reply]

Push Back[edit]

I guess there is no mention of push back in this article. Push back is found in many Prolog systems DCG implementations. It has the following syntax:

  NT, PH_1 --> PH_2

The meaning is that after parsing the phrase PH_2, the phrase PH_2 is pushed back on the difference list. This allos for checking termimals without consuming them.

For example:

  starts_with_digit, [X] --> [X], {digit(X)}.

Janburse (talk) 10:08, 26 November 2011 (UTC)[reply]

Negation[edit]

The article does also not mention negation in DCG or conditionals in DCG. Many Prolog system DCG implementations nowadays provide these constructs.

Nagation has the following translation:

  p --> \+ q.

Is translated to:

  p(X,Y) :- \+ q(X,_), X=Y.

So again the difference list is not consumed. But this time a condition is evaluated, and when it fails parsing proceeds.

Janburse (talk) 10:08, 26 November 2011 (UTC)[reply]

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