backtracking over streams?

[Jean-Paul Berger]

hi,


i write a program that needs to backtrack over streams. but my predicate 
only backtracks over one instance and doesn't work for backtracking over two 
clauses.

is there any way to get the desired effect? (prolog or c - doesn't matter, 
which).


thank you.



PS: my tries so far...:

% back/2: back(+Stream_Or_Alias, ?Character_Code_List)

back(S, [A|B]) :-
        peek_code(S, A), A \= -1,
        get_code(S, A),
        (back(S, B) -> true ; ( unget_code(S, A), fail )).
back(_,[]).



% DOESN'T WORK:

identifier(A, B) :-
       character(A, C),
       identifier(A, D),
       atom_concat(C, D, B).
identifier(A, B) :-
       character(A, B).
character(A, B) :-
       lower(A, C),
       name(B, [C]).
lower(A, B) :-
       back(A, [B]),
       member(B, [97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 
109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122]).


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