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16.7 An example using impurity

The following example illustrates how a pure predicate may be implemented using impure code. Note that this code is not reentrant, and so is not useful as is. It is meant only as an example. 

:- pragma foreign_decl("C", "#include <limits.h>").
:- pragma foreign_decl("C", "extern MR_Integer max;").

:- pragma foreign_code("C", "MR_Integer max;").

:- impure pred init_max is det.
:- pragma foreign_proc("C",
    init_max,
    [will_not_call_mercury],
"
    max = INT_MIN;
").

:- impure pred set_max(int::in) is det.
:- pragma foreign_proc("C",
   set_max(X::in),
   [will_not_call_mercury],
"
    if (X > max) max = X;
").

:- semipure func get_max = (int::out) is det.
:- pragma foreign_proc("C",
    get_max = (X::out),
    [promise_semipure, will_not_call_mercury],
"
    X = max;
").

:- pragma promise_pure(max_solution/2).
:- pred max_solution(pred(int), int).
:- mode max_solution(pred(out) is multi, out) is det.

max_solution(Generator, Max) :-
    impure init_max,
    (
        Generator(X),
        impure set_max(X),
        fail
    ;
        semipure Max = get_max
    ).