[Perl5-syntax] How to test for a role in a method signature

[Reverend Chip]

On 3/4/2011 3:06 AM, Piers Cawley wrote:
> On Fri, Mar 4, 2011 at 10:29 AM, Reverend Chip <rev.chip at gmail.com> wrote:
>> On 3/4/2011 1:57 AM, Piers Cawley wrote:
>>> On Fri, Mar 4, 2011 at 7:16 AM, Reverend Chip <rev.chip at gmail.com> wrote:
>>>> On 3/3/2011 10:32 PM, Piers Cawley wrote:
>>>>> Personally, I prefer the flow of 'Typed $parameter does coerce', where
>>>>> the constraint declaration is always to the left of the parameter name
>>>>> and the right hand side is for declaring stuff about how the parameter
>>>>> is initialized. I would encourage anyone wanting to constrain by type
>>>>> to declare a type along the lines of "subtype DoesRole, as role_type
>>>>> 'Role'" ...
>>>> No, I don't buy it.  The "isa" and "does" tests are so fundamental and
>>>> common that forcing people to type that much to get the test would
>>>> violate Huffman.
>>> That's why I think that the better solution is to delegate to the
>>> package to find out if a particular value satisfies it. If the package
>>> is a class, then obviously it does an 'isa' test, if a role, then it
>>> does a 'does' check and, if a type constraint, it uses the type's
>>> constraints.
>> And for combining?  You only get one name to the left.
> If you _really_ want to ensure that a parameter isa Foo and also does
> Bar then you need to rethink your architecture. Or hide the pain, bite
> the bullet and make a custom type.
>
> Easy things should be easy. Hard things should be possible. Stupid
> things should be discouraged.

I was thinking of does Foo and does Bar, actually.  But there is no way
at compile time to know which test will apply, so your proposal is
slower at runtime.  That's a killer.