Re: Signature Named Parameters and `meta` Queries

On Wed, 12 Nov 2025 17:40:53 +0000
Harald Jörg <[email protected]> wrote:

> I would prefer if min_args and max_args count positional arguments
> only, and named parameters are only available using the named_params
> method. Consider the following signature:

Oh, we already have "mandatory_params" and "optional_params" accessors
that count the positional ones. The point with min/max is more to
indicate that numbers outside of this range are definitely, absolutely
errors.

>     sub from_file ($class, :$path) { ... }
> 
> Having "min" and "max" suggests that any number inbetween is valid to
> be passed by the caller.  This is not the case if you count named
> parameters: The number of parameters has be odd, however often you
> provide a 'path => somewhere,'.

Mmm, yes the arity is already an interesting question. But that also
comes up without named ones when you consider slurpy hashes.

> I admit that I currently have no use for meta operations on signatures
> (OTOH, I would very much welcome introspection of classes).

Hmmmyes I should look into that sometime.

I'm hesitant to add *too* much more to the `meta` API shape right now
though until some more basic questions have been answered - like the
problems surrounding get-or-fail, vs get-or-undef semantics on a lot of
the accessors.

> My
> current use of named parameters is for alternate constructors, so I
> have always one mandatory positional argument and then some named
> ones.  Named parameters in the signature take a lot of runtime
> validation work from me compared to the "old" signature
> ($class,%params).

Speaking of alternate constructors, you may wish to be aware of

  https://metacpan.org/pod/Object::Pad#%3Alexical_new

The intention there being you can hide the default-provided constructor
and wrap it in your own provided methods. In fact if you're a bit
subtle about it you can even provide a visible method named `new` ;)

  class PointOverridingNew :lexical_new
  {
     method new :common
     {
        my ( $x, $y ) = @_;
        return new( __PACKAGE__, x => $x, y => $y );
     }
  
     field $x :param :reader;
     field $y :param :reader;
  }

I'm intending to bring `:abstract` and `:lexical_new` into the core
feature class work soon, once I have slightly improved the overall
situation with attributes generally.

-- 
Paul "LeoNerd" Evans

[email protected]
http://www.leonerd.org.uk/  |  https://metacpan.org/author/PEVANS