prolog - parsing an expression using DCG -


i try parse list of identifiers dcg fail it,

example

                          ::=                             |                           __|____                          |       |                         id     id,'(',ids,')'.                                        _|_                                       |   |                                      id   id,

-

ids ::= id. ids ::= id(id1,id2,...).

example of identifier verify

  exp1 = 'id_0'   exp3 = 'id_1(i)'   exp2 = 'id_2(i1,i2)'

code edit

id([id]) --> [id]. id([id|ids]) --> [id,'('], ids(ids), [')'].  ids([id]) --> [id]. ids([id|ids]) --> [id], ids(ids).

someone tell me problem ?

let first see currently describing (for example) id//1, using most general query:

?- phrase(id(is), ls). = ls, ls = [_g884] ; = [_g884|_g885], ls = [_g884, '(', ids(_g885), ')'].

trying out general query way find out more relation. in case, note 2 important things:

  1. regarding list ls of tokens, first solution more general intended. no clause add program (while preserving ) can remove flaw. therefore, need make first rule more specific.
  2. the tokens described second answer not match grammar, because contain concrete token ids(_). points fact intermingling dcg rule names tokens want describe.

for start, let first focus describing list of tokens want describe. simplifies code somewhat, since not have deal many things @ once.

i suggest start following:

ids --> [id]. ids --> [id,'('], ids_, [')'].  ids_ --> ids, more_ids_.  more_ids_ --> []. more_ids_ --> [,], ids_.

notice helpful pattern allows specify "more of same", either nothing or comma and then "the same".

i suppose make more general, , example accept other identifiers besides concrete token id. place clear code above, , leave easy exercise.

notice can use see whether describing only intended lists of tokens:

?- length(ls, _), phrase(ids, ls). ls = [id] ; ls = [id, '(', id, ')'] ; ls = [id, '(', id, ',', id, ')'] ; ls = [id, '(', id, '(', id, ')', ')'] ; etc.

i using length/2 fair enumeration.

once have made sure describing , lists of tokens intended, can start relate such lists of tokens other prolog terms. involve (=..)/2 , other relations between terms.


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