How can I catch the output to the console in Prolog to verify it?

Question

Suppose I have a hello_name.pl:

greeting (Name): -
   write ('hello'),
   write (Name),
   writeln ('!').

And I want to put in my plunit something like

catch_output (greeting ('Moncho'), ConsoleOutput),
  assertion ('hello Moncho!' =:= ConsoleOutput).

Just as an addendum, do not use `write` (it is meant so serialize-out terms), use `format`. I have prepared a little overview on "printing in Prolog" [in this gist](https://gist.github.com/dtonhofer/20bd01f68a924912771d8405fca66a09) (Work in progress). — David Tonhofer, Jan 19 '20 at 20:59
@DavidTonhofer thank you sir, I added your link in the [Codewars kata I've created](https://www.codewars.com/kata/57036f007fd72e3b77000023) — Ruslan López, Jan 21 '20 at 01:52
Of interest: [Help with set_prolog_IO/3](https://swi-prolog.discourse.group/t/help-with-set-prolog-io-3/1798) — Guy Coder, Jan 29 '20 at 10:25

score 3 · Accepted Answer · edited Jan 21 '20 at 01:48

If you are using swi-prolog

See: with_output_to/2

Note: with_output_to/2 is implemented using C in SWI-Prolog so is not portable as Prolog code.

?- with_output_to(string(Output),(write('hello'),write('Rusian'),write('!'))), 
   assertion( Output == "helloRusian!").

With corrections to your code and using SWI-Prolog unit tests

greeting(Name) :-
   write('hello'),
   write(Name),
   writeln('!').

:- begin_tests(your_tests).

test(001, Output == 'helloMoncho!\n') :-
    with_output_to(atom(Output), greeting('Moncho')).

:- end_tests(your_tests).

How can I catch the output to the console in Prolog to verify it?

1 Answers1