So you're learning Mercury lang and you have a predicate with multi
determinism. Maybe it looks something like this:
:- pred school(string:: in, string:: out) is multi.
fish(Fish, ParticularFish) :-
( append("One ", Fish, ParticularFish)
; append("Two ", Fish, ParticularFish)
; append("Red ", Fish, ParticularFish)
; append("Blue ", Fish, ParticularFish)
).
Which has four solutions for any given string. "One Fish", "Two Fish", "Red Fish", "Blue Fish".
Flash forward. Now you need to deal with I/O, and I/O can't handle
multiple solutions because in order to maintain purity Mercury can't backtrack
within predicates that do I/O. We need to gather the solutions before returning
them to the main predicate.
Luckily there is the solutions/2 predicate which does exactly this. Here's an
example:
School = solutions(fish("Fish"))
Notice how we omitted the second argument? That's called currying; you can read more here.
main(!IO) :-
io.read_line_as_string(Result, !IO),
(
Result = eof,
io.format("bye bye...\n", [], !IO)
;
Result = ok(NewlineString),
String = string.strip(NewlineString),
School = solutions(fish(String)),
io.format("%s\n", [s(write_list(School))], !IO),
main(!IO)
;
Result = error(ErrorCode),
io.format("%s\n", [s(io.error_message(ErrorCode))], !IO)
).
If you find any errors, please contact me!