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

[Piers Cawley]

On Fri, Mar 4, 2011 at 7:24 PM, Reverend Chip <rev.chip at gmail.com> wrote:
> 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.

And down that road lies C++. Let the constraint (be it a role, a class
or a moose type) decide what tests to run. You pay the cost of a
method call and get more convenient syntax. How do you plan to signal
that a parameter should be coerced?