问题
?- length(L,25).
L = [_G245, _G248, _G251, _G254, _G257, _G260, _G263, _G266, _G
269|...].
If I use write(L) following the length predicate then the interpreter prints the list twice, one expanded and the other not.
回答1:
There is a limit on the depth to prevent too long output. You can change it with set_prolog_flag/1.
?- length(L, 25).
L = [_G257, _G260, _G263, _G266, _G269, _G272, _G275, _G278, _G281|...].
?- current_prolog_flag(toplevel_print_options, V).
V = [quoted(true), portray(true), max_depth(10), priority(699)].
?- set_prolog_flag(toplevel_print_options, [quoted(true), portray(true), max_depth(100), priority(699)]).
true.
?- length(L, 25).
L = [_G257, _G260, _G263, _G266, _G269, _G272, _G275, _G278, _G281, _G284, _G287, _G290, _G293, _G296, _G299, _G302, _G305, _G308, _G311, _G314, _G317, _G320, _G323, _G326, _G329].
Edit: You can also remove the limit completely by removing it from the options list.
来源：https://stackoverflow.com/questions/1525995/how-to-expand-a-resulting-list-in-swi-prolog