perl6: how to specify multiple requirements for a parameter of a function?

Question

I have a special function that takes a list, each member of the list must satisfy multiple requirements. How do I set this up in a perl6 function?

sub specialFunc(List $x) {};

$x is a list # easy, List $x, but what about the following:
each member of $x is numeric
each member of $x is positive
each member of $x is greater than 7
each member of $x is odd number
each member of $x is either the square or the cube of an even number plus 1;

score 8 · Accepted Answer · answered Dec 10 '16 at 20:59

The Perl 6 type system is not flexible enough to express such constraints declaratively, but you can add a where clause to your parameter to check incoming arguments against a custom expression.

For clarity, I'd factor out the expression for testing each number into a subset:

subset SpecialNumber of Numeric where {
       $_ > 7                        # (3), (4)
    && $_ !%% 2                      # (5), since "odd" implies "not even"
    && .narrow ~~ Int                # (5), since "odd" implies "integer"
    && ($_ - 1) ** (1/2 | 1/3) %% 2  # (6)
}

sub specialFunc(List $x where .all ~~ SpecialNumber ) {
    ...
}

You could go one step further, and factor out the whole where clause into a subset:

subset SpecialList of List where .all ~~ SpecialNumber;

sub specialFunc(SpecialList $x) {
    ...
}

PS: I think your requirement (5) may be redundant, since requirement (6) seems to only satisfy odd numbers anyway, but I'm not big on number theory so I'm not sure.

Thank you very much, smls !!! I agree, (6) implies (5) which is redundant. Thank you again ! — lisprogtor, Dec 12 '16 at 09:01
or, you could type an Array... ```my SpecialNumber @a; sub specialFunc(SpecialNumber @a) { ... }``` — librasteve, Jul 06 '23 at 18:07

perl6: how to specify multiple requirements for a parameter of a function?

1 Answers1