Read a file line by line in Prolog

Question

I\'d like to read a plain text file and apply a predicate to each line (the predicates contain write which does the output). How would I do that?

Answer 1

There are kind of more possible in number and more reasonable in performance solutions, to get uninterpreted i.e plain text lines from a file:

SWI-Prolog:

read_line(S, X) :- 
   read_line_to_codes(S, L), 
   read_line2(L, X).

read_line2(end_of_file, _) :- !, fail.
read_line2(L, X) :-
   atom_codes(X, L).

Jekejeke Prolog:

:- use_module(library(stream/console)).

Here are some timings, reading a file of 655 lines:

test :-
   open('', read, Stream),
   test(Stream),
   close(Stream).

test(Stream) :-
   read_line(Stream, _), !,
   test(Stream).
test(_).

SWI-Prolog:

̀?- time((between(1,100,_), test, fail; true)).
% 328,300 inferences, 0.125 CPU in 0.143 seconds (88% CPU, 2626400 Lips)
true.

Jekejeke Prolog:

?- time((between(1,100,_), test, fail; true)).
% Up 121 ms, GC 2 ms, Thread Cpu 94 ms (Current 05/07/19 17:19:05)
Yes

I guess a SWI-Prolog solution that reads into a string instead into an atom could be faster. But in the above we compare atom against atom reading.