Read from file and display the content using Prolo

Question

i am a newbie in prolog, i want to read a file which actually contains CLASS Definition - using Prolog DCG Rule. But i am stuck in between now..

My input text (linessample.txt)

class component {
        attributes
          Real V, I, R;
        constraints
          V = I * R;
        constructors component(V1, I1, R1) {
          V = V1; I = I1; R = R1;
        }
} 

I want to read the above sample text using DCG Rule in prolog... I have written one sample code.. but i am not getting the first word "class" in the output

Code

    :- use_module(library(pio)).
    %classrule(Z) -->class,letter(X),letters(L),{name(Z,[X|L])}.
    classrule(Z) -->"class ",classname(X),"{",{name(Z,X)}.
    classname([X|L])-->letter(X),letters(L).
    letters([X|L])-->letter(X),!,letters(L).
    class-->"class".
    letters([])-->[].
    letter(X)-->[X], {alpha(X)}.
    alpha(X) :- X > 64, X < 91.
    alpha(X) :- X > 96, X < 123.

how to run this

    phrase_from_file(classrule(Z),'linesample.txt').  

Answer 1

There are two main problems with your program: 1. You do not allow for a space to occur between classname/1 and "{". Since in practice multiple spaces (and horizontal tabs) may occur here, I have used whites//0 from library dcg/basics. 2. phrase_from_file/2 tries to parse the entire input document, whereas your grammar only covers the first line (i.e., the class name). This is solved by skipping the rest of the file. I use '...'//0 and then eos//0 for this.

Then there are some minor things: 1. Clauses of letters//1 are not placed consecutively in the code file. I have relocated one clause but you can also add the declarations :- discontiguous(letters//1). at the top of your program. 2. I have used code_type/2 to check for alphabetic characters.

The resultant code, updated after useful comments from mat:

:- use_module(library(dcg/basics)).
:- use_module(library(pio)).

classrule(Z) -->
  "class",
  whites,
  classname(X),
  whites,
  "{",
  {name(Z,X)},
  ... .

classname([H|T])-->
  letter(H),
  letters(T).

letters([H|T])-->
  letter(H), !,
  letters(T).
letters([])--> [].

letter(X)-->
  [X],
  {code_type(X, alpha)}.

... --> [].
... -->
  [_],
  ... .

Example of use:

?- phrase_from_file(classrule(X), 'linesample.txt').
X = component