Use `print` and `~p` instead of `write_term` and `~w`

Discussion
1

Basically, I want the terms as answers and the terms for printing to be the same.

The default values of the flag answer_write_options is:

?- current_prolog_flag(answer_write_options, Value).
Value = [quoted(true), portray(true), max_depth(10), spacing(next_argument)].

The default value of the flag print_write_options is:

?- current_prolog_flag(print_write_options, Value ).
Value = [portray(true), quoted(true), numbervars(true)].

Notice that these two default values are not exactly the same. So yes, by default the terms as answers and the terms for printing are not the same.

For example:

?- Term = '$VAR'(10), print(Term).
'$VAR'(10)
Term = K.

?- Term = foo(1,2,3), print(foo(1,2,3)).
foo(1,2,3)
Term = foo(1, 2, 3).

?- Term = [1,2,3,4,5,6,7,8,9,10,11,12,13], print(Term).
[1,2,3,4,5,6,7,8,9,10,11,12,13]
Term = [1, 2, 3, 4, 5, 6, 7, 8, 9|...].

But we can make them be the same! Just setting them in the init.pl:

For example,

:- set_prolog_flag(print_write_options, [portray(true), quoted(true)]).
:- set_prolog_flag(answer_write_options, [portray(true), quoted(true)]).

Then everything is OK:

?- Term = '$VAR'(10), print(Term).
K
Term = K.

?- Term = foo(1,2,3), print(foo(1,2,3)).
foo(1,2,3)
Term = foo(1,2,3).

?- Term = [1,2,3,4,5,6,7,8,9,10,11,12,13], print(Term).
[1,2,3,4,5,6,7,8,9,10,11,12,13]
Term = [1,2,3,4,5,6,7,8,9,10,11,12,13].

However, there is no flag for write_term/2, i.e. you cannot set default Options for write_term/2 (at least I didn’t find…). So when you want to write a term, you have to do, for example,

?- Term = '$A'(0), write_term(Term, [quoted(true)]).
'$A'(0)
Term = '$A'(0).

or

?- Term = '$A'(0), format("~W~n", [Term, [quoted(true)]]).
'$A'(0)
Term = '$A'(0).

This is obviously not convenient. The better way is using print/1 and format/3 with ~p, because they can have default values:

?- Term = '$A'(0), print(Term).
'$A'(0)
Term = '$A'(0).

or

?- Term = '$A'(0), format("~p~n", Term).
'$A'(0)
Term = '$A'(0).

BTW, I saw many people using writeln/1, write_term/2, ~w in format/3, etc for printing, which is not ideal IMO, because it would make the answer terms inconsistent with the printed terms (notably for quoted atoms).

This is what I learnt today.

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2

Whether you want space on the next argument or not is personal taste. Omitting the depth limit for answers is not something I’d not recommend. You typically do not want a list with 1M elements printed. print/1 is mainly for debugging purposes. Using a depth limit there might actually be a good idea as well. write/1, as I have argued before, is a weird predicate. t doesn’t quote and thus the result cannot be read by Prolog. format/3 makes more sense when emitting something for human consumption. write_term/2 is useful as it provides a lot of control on the output, most of which proved to be useful in particular contexts.

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