So you're learning Mercury lang and you have a predicate with
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
Luckily there is the
solutions/2 predicate which does exactly this. Here's an
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!