CODEWITHSUNDEEP
parsingprologdcg
nash
nash

Reputation: 305

Prolog - parsing functions with DCG

I need to parse a string representing function like this:

<fsignature>"(" <term1>, <term2> ... <termn>")"

The function' signature and terms also have to be controlled further for the function to be accepted. I've written this DCG in Prolog:

fsign --> {is_leg(A)}, [A].
terms --> (funct, funct; terms).
terms --> (terms, terms; term).
term --> {is_term(T)}, [T].

But it doesn't seem to work, when I try to use phrase(funct, [foo, "(", a, a, ")"]). it goes into overflow. is_leg just checks if the string is legal (a string starting with a character), while is_term should check if the term is a literal (either a constant, a variable or a function).

What is that it's not working? I figure, probably the variables - should I put them as arguments of the nonterminals?

Thanks for any help.

Upvotes: 1

Views: 261

Answers (1)

lurker
lurker

Reputation: 58324

If your expressions all look like this:

<fsignature> "(" <term1> <term2> ... <termn> ")"

Then writing this out in terms of DCG, it should look something like this (minus any string validation predicates you're suggesting):

expression --> fsignature, ['('], terms, [')'].
fsignature --> ...      % whatever fsignature looks like
terms --> term.         % "terms" is a single term, or...
terms --> terms, term.  %   ... more terms followed by a single term.
term --> ...            % whatever a term looks like

You can also write the definition of terms as:

terms --> term | terms, term.

Note that the non-recursive definition of terms occurs before the recursive one. Also, the definition for terms above assumes that you must have at least one term (this requirement wasn't stated in your question).

Upvotes: 3

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