Re: Distinguish between missing and undefined arguments with subroutine signatures

[LanX]

> Is there a common idiom how to handle such case with signatures?

The rule of thumb is to use a default which is defined. °

Using undef as default should be redundant and will force you to check @_ - if still possible (untested)

Sorry, your code is longish (aka TLDR), if I didn't get it right, please consider condensing it to the relevant part.

This question could be actually very relevant if there is a real use case! P5P is considering to drop the population of @_ when signatures are used.

update

°) not

# Standard signature with optional second arg.

sub sig_std ($what, $arg=undef) {

[dave_the_m]

P5P is considering to drop the population of @_ when signatures are used

But only after alternative facilities for argument instrospection have been provided. See the proposal on the p5p mailing list.

In particular, in

sub foo($x,  ??$has_y, $y = 0) { ... }

$has_y is true if and only if a second arg is passed to the sub.

Dave.

[jo37]

sub foo($x,  ??$has_y, $y = 0) { ... }

Great! That's it. (Or better: Will be)

[jo37]

The use case I have in mind is a combined getter/setter for a perl object. Return the value when called without an additional arg, set the value when called with an arg. A call with an undefined arg clears the attribute - just like in my example.

Maybe that's not a real world use case - I'm just playing with things.

[LanX]

> combined getter/setter ... Return the value when called without an additional arg,

from my experience are mutators written in a way to always return the value.

> A call with an undefined arg clears the attribute - just like in my example.

OK ... but in that case why do you have a default value at all?

Anyway, if you want to allow to set to undef and don't wanna introspect @_ , then you're stuck in a kind of semipredicate problem.

Such problems can be solved in Perl by setting a new "impossible"° object MISSING as default, like

use constant MISSING => bless {}, "MyPackg::__Missing__"

and later

sub mutator ($value = MISSING) { ...BODY... }

(untested)

If you see that an arguments equals to that "impossible" value blessed into your own private namespace below "MyPackg", you can be sure that it wasn't used by accident.

HTH! :)

update

clearer rewording, clearer code

°) of course it's possible that such a value is used, but extremely unlikely without intend.

[jo37]

use constant MISSING => bless {}, "MyPackg::__Missing__"

That's a very good solution.

Thx.

[jo37]

OK ... but in that case why do you have a default value at all?

I don't want a default value. To my understanding it is needed for an optional value argument in a signature.