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

[Piers Cawley]

On Mon, Mar 7, 2011 at 9:08 AM, Michael G Schwern <schwern at pobox.com> wrote:
> On 2011.3.7 6:46 PM, Buddy Burden wrote:
>> Now, I'm still concerned about Piers' point that, if you were to have
>> MXMS code and you wanted to convert over to ours--and why wouldn't
>> you? it's cooler, less buggy, and it runs faster--you really wouldn't
>> want to have to wait and find out that you missed a "does coerce"
>> until runtime.  So it occurs to me: I'm building my cache of type
>> constraints at runtime, as they're called.  But I don't _have_ to do
>> that, right?  Why not build the cache at compile-time, as the types
>> are parsed?  That way, any unrecognized type gets reported
>> immediately, at compile-time.  So your "does coerce" spits out an
>> error such as:
>
> The downside of resolving types at compile time is if a... whatever[1] isn't
> loaded before your method compiles it will be treated as something you don't
> expect.  And we don't want to force people to load classes at compile time
> just so the types resolve at compile time.  That means they can't be run
> selectively at runtime.

Exactly my thought. I've certainly used class constraints on
parameters in classes that don't load said class because doing so
would create a circular dependency

> A simpler approach would be for MXD to special case "does coerce" in parsing.
>  If it sees a role called "coerce" while parsing it warns.  Ta da!

Works for me. Though should it still warn if, for some reason, there's
a role 'coerce' already loaded?

I like the proposal I saw floating around somewhere of using 'will
coerce' as the 'right' syntax for indicating coercion. It seems to fit
the place better than either 'is' or 'does'.