CODEWITHSUNDEEP
parsingprologgrammardcg
Caspian Ahlberg
Caspian Ahlberg

Reputation: 976

How do I create a DCG rule inverse to another in Prolog?

I am writing a Commodore BASIC interpreter in Prolog, and I am writing some DCGs to parse it. I have verified the DCGs below to work except for the variable one. My goal is this: for anything which isn't a boolean, integer, float, or a string, it's a variable. However, anything that I give it via phrase just results in no.

bool --> [true].
bool --> [false].
integer --> [1]. % how to match nums?
float --> [0.1].
string --> [Str], {atom_chars(Str, ['"' | Chars]), last(Chars, '"')}.
literal --> bool; integer; float; string.

variable --> \+ literal.

I ran a stack trace like this (with gprolog)

main :- trace, phrase(variable, [bar]).

Looking at this, I cannot figure out why variable fails, given that it fails for each case in literal. I'm guessing that the error is pretty simple, but I'm still stumped, so does anyone who's good at Prolog have an idea of what I'm doing wrong?

| ?- main.
The debugger will first creep -- showing everything (trace)
      1    1  Call: phrase(variable,[bar]) ? 
      2    2  Call: variable([bar],_321) ? 
      3    3  Call: \+literal([bar],_348) ? 
      4    4  Call: literal([bar],_348) ? 
      5    5  Call: bool([bar],_348) ? 
      5    5  Fail: bool([bar],_348) ? 
      5    5  Call: integer([bar],_348) ? 
      5    5  Fail: integer([bar],_348) ? 
      5    5  Call: float([bar],_348) ? 
      5    5  Fail: float([bar],_348) ? 
      5    5  Call: string([bar],_348) ? 
      6    6  Call: atom_chars(bar,['"'|_418]) ? 
      6    6  Fail: atom_chars(bar,['"'|_418]) ? 
      5    5  Fail: string([bar],_348) ? 
      4    4  Fail: literal([bar],_348) ? 
      3    3  Exit: \+literal([bar],_348) ? 
      2    2  Exit: variable([bar],[bar]) ? 
      1    1  Fail: phrase(variable,[bar]) ? 

(2 ms) no
{trace}

Upvotes: 3

Views: 151

Answers (2)

Isabelle Newbie
Isabelle Newbie

Reputation: 9378

To expand a bit on the other answer, the key problem is that a DCG rule like \+ literal does not consume items from the input. It only checks that the next item, if any, is not a literal.

To actually consume an item, you need to use a list goal, similarly to how you use a goal [1] to consume a 1 element. So:

variable -->
    \+ literal,  % if there is a next element, it's not a literal
    [_Variable]. % consume this next element, which is a variable

For example:

?- phrase(variable, [bar]).
true.

?- phrase((integer, variable, float), [I, bar, F]).
I = 1,
F = 0.1.

Having that singleton variable _Variable is a bit strange -- when you parse like this, you lose the name of the variable. When your parser is expanded a bit, you will want to use arguments to your DCG rules to communicate information out of the rules:

variable(Variable) -->
    \+ literal,  
    [Variable].

For example:

?- phrase((integer, variable(Var1), float, variable(Var2)), [I, bar, F, foo]).
Var1 = bar,
Var2 = foo,
I = 1,
F = 0.1.

Upvotes: 3

Peter Ludemann
Peter Ludemann

Reputation: 1005

You can detect a string of integers like this (I've added an argument to collect the digits):

integer([H|T]) --> digit(H), integer(T).
integer([]) -->  [].
digit(0) --> "0".
digit(1) --> "1".
    ...
digit(9) --> "9".

As for variable -- it needs to consume text, so you'd want something similar to integer above, but change digit(H) to something that recognizes a character that's part of a "variable".

If you want further clues (although sometimes using slightly advanced tricks): https://www.swi-prolog.org/pldoc/man?section=basics and the code is here: https://github.com/SWI-Prolog/swipl-devel/blob/master/library/dcg/basics.pl

Upvotes: 3

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