named-param-with-constraint

test-param.p6

#!/usr/bin/env perl6

foo;

sub fmt($fmt) {
  my token fmt { ^ :i a|b $ }
  !$fmt.defined || $fmt ~~ &fmt
}

sub foo {
    if $fmt.defined {
	say "fmt = $fmt"
    }
    else {
        say "fmt = undefined"
    }
}