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Re: arity checking (was Re: PSC #049 2022-01-07)

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Dave Mitchell
January 21, 2022 17:49
Re: arity checking (was Re: PSC #049 2022-01-07)
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On Fri, Jan 21, 2022 at 06:29:27PM +0100, demerphq wrote:
> So consider, I own a framework whose api is that I accept a callback from
> the caller. I tell the caller that my api will call his sub with 1
> argument, lets say "key".

There's the crunch. If your framework promises to call always with one
arg, then if you call the callback with 2 args, you've broken your

If you've promised 1 arg, then the user of your framework should be able to
pass either of these subs:

    sub callback { croak unless @_ == 1; my ($x) = @_ }
    sub callback ($x) { }

If the framework promises it will pass at *least* one arg, then these
should both be valid:

    sub callback { croak unless @_ >= 1; my ($x) = @_ }
    sub callback ($x, @) { }

If the framework says you can't trust it in any fashion, then both of
these would do:

    sub callback { my ($x) = @_ }
    sub callback ($x=undef, @) { }

There is no fundamental difference between each pair of subs. In the first
pair, the writer of the sub has made a conscious choice to write a
subroutine with strict arity checking, both signatured and unsignatured

Now you can of course argue that signatures make it *easier* to
inadvertently write subroutines which have arity checking when with
hindsight it would have been easier to migrate to a new version of the
framework if you had disabled arity checking. But its not a fundamental
design problem with signatures.

If a framework author wants to break a promise about arity, they could
always call it with an eval:

    if (defined $extra) {
        eval { $self->$callback($x, $y, $extra) };
        goto fallback if $@ =~ /Too many arguments/;
    else {
        $self->$callback($x, $y);

which is as much (or little) hacky as introspecting the callback to see if
its a sub with a declared arity range.

You're only as old as you look.

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