Read all lines to a list

file_lines(File, Lines) :-
    setup_call_cleanup(open(File, read, In),
       stream_lines(In, Lines),
       close(In)).

stream_lines(In, Lines) :-
    read_string(In, _, Str),
    split_string(Str, "\n", "", Lines).

This loads all lines to memory as strings. If we use the Unix dictionary words:

?- file_lines("/usr/share/dict/words", Lines).
Words = ["A", "a", "aa", "aal", "aalii", "aam", "Aani", "aardvark", "aardwolf"|...].

Backtrack over lines in a file

file_line(File, Line) :-
    setup_call_cleanup(open(File, read, In),
        stream_line(In, Line),
        close(In)).

stream_line(In, Line) :-
    repeat,
    (   read_line_to_string(In, Line0),
        Line0 \== end_of_file
    ->  Line0 = Line
    ;   !,
        fail
    ).

This will backtrack over consecutive lines in the input. If we use the Unix dictionary words:

?- file_line("/usr/share/dict/words", Word).
Word = "A" ;
Word = "a" ;
Word = "aa" ;
Word = "aal" ; % and so on

This doesn't load all lines to memory, so it can be used with very large files.

See also

- read_line_to_codes/2 to read to a code list instead of a string

- read_string/5 for more control over the separator and stripping