Both the toplevel query/answer loop as the debugger abbreviate long complex terms. They do this to avoid endless pages of output. In fact, they write using write_term/3 which takes an option-list as argument. The option list for answers printed by the Prolog toplevel is in the prolog-flag toplevel_print_options and the one for the debugger is in debugger_print_options. Initially both have the value given below:

?- current_prolog_flag(answer_write_options, X).

X = [quoted(true), portray(true), max_depth(10),
     spacing(next_argument)].

The option max_depth(10) says anything nested more then 10 levels should be written as ...

What to do?

If the system prints an answer that is abbreviated and you want to see more, type + and the system will increase the max_depth limit 10 fold for printing the answer: (the user pressed + at the place the diagram says [max_depth(100)]). Note the ; true. This is used to introduce non-determinism that makes Prolog wait after the answer. If the answer is deterministic, Prolog prints it with the default settings and prompts for the next command.

?- atom_chars(goodbye_prolog, X) ; true.

X = [g, o, o, d, b, y, e, '_', p|...] [max_depth(100)]

X = [g, o, o, d, b, y, e, '_', p, r, o, l, o, g]

Use the - command to reduce max_depth 10 fold.

The toplevel and debugger also have the commands w and p to disable/enable the user portray/1 hook. The portray/1 hook may be defined by the user to print complex terms in a more friendly way.

Changing default?

Add a set_prolog_flag/2 directive in your prolog personal initialisation file (see PlInitialisation.md) to change the default the above mentioned prolog flags. E.g. to disable abbreviation, use:

?- set_prolog_flag(answer_write_options,
                   [ quoted(true),
                     portray(true),
                     spacing(next_argument)
                   ]).

See also

- portray_text/1 for displaying lists of character codes as strings.