quick.p6

module Test::QuickCheck {

  multi gen(Int) { 1000000.rand.Int }

  our sub verify-one( &f, &predicate ) {
    my @params = &f.signature.params.map: -> $p {
      # say "Param: {$p.gist}";
      gen($p.type)
    };
    my $result = &f(|@params);
    # say "Result: {$result.gist}";
    my $is-ok = so &predicate($result);
    ($is-ok, $result);
  }

  our sub verify(&f, &predicate) {
    for ^1000 {
      # return False unless verify-one(&f, &predicate);
      my ($is-ok, $result) = verify-one(&f, &predicate);
      if !$is-ok {
        return (False, $result);
      }
    }
    True;
  }

}

sub is-even($n) { $n %% 2 }

sub timestwo(Int $n) { $n * 2 }

sub timesthree(Int $n) { $n * 3 }

say Test::QuickCheck::verify(&timestwo, &is-even);

say Test::QuickCheck::verify(&timesthree, &is-even);

say Test::QuickCheck::verify(&timesthree, * %% 2);