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RE: A modest question (Damian, see last para please?)

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Austin Hastings
January 7, 2004 23:21
RE: A modest question (Damian, see last para please?)
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Jonathan Lang [] wrote:
> Austin Hastings wrote:
> > Indeed. I like the idea of dynamic anonymous roles -- it's more
> > behavioral than anything else.
> >
> >  sub print_it ($thingie must stringify()) {...}
> >
> > Definitely gets down to the lowest level quickly, which is nice. Even
> > nicer is the ability to use this sort of requirement as kind of an
> > advanced signature: declare exactly what you're going to do. (In other
> > words, your signature may say
> >
> >   sub foo(Object $o) {...}
> >
> > because you want to accept anything in the hierarchy. But it's nice to
> > extend it with
> >
> >   sub foo(Object $o must stringify() must isa() must typeof()) {...}
> Valid, if wordy.  Roles remain useful in that they provide a more concise
> way of handling this if you want it - if you've got a dozen routines that
> all C<must isa() must typeof()>, you might be better off defining a role
> that makes those demands, and then just use it.

Sure, I agree. But when you want to specify exactly what you're doing (sort
of like call-by-contract, I guess) you can get really detailed. When you
have a dozen common cases, define an inferred role. But if any object with
methods A, Q, and Z are going to be acceptable, why not?

> > This kind of granularity does kind of imply a JavaScript-like ability to
> > compose objects, too, no? (If you can compose requirements atomically,
> > why not compose capabilities, too?)
> >
> >   my $photon does Particle does Wave {...} = spark();
> That's where C<but> comes in:
>    my $photon but does Particle does Wave {...} = spark();
> would be equivelent to something like
>    class _anonymous_photon does Particle does Wave {...}
>    my _anonymous_photon $photon = spark();

Is that ['but'] really necessary?

> > > Also: in the first of these two, would classof($thingie) actually have
> > > to have Stringify as a role, or would it be reasonable to instead say
> > > that classof($thingie) must meet Stringify's demands?  The latter
> > > would require more work on the compiler's part, but would be
> > > considerably more flexible.
> >
> > I prefer the latter. I want to be able to compose requirements on the
> > way. I certainly don't want to have to rewrite the core libraries (or
> > extend them all) just to mix in an interface role that they already
> > satisfy.
> In principle, I agree with you; in practice, it may not be workable.

Verification should be just as easy for this as for a typed referenced
parameter. I don't see why it wouldn't be workable.

> > >  Perhaps "Stringify $thingie" requires that the Stringify role must
> > > actually be used, while something like "$thingie like Stringify" would
> > > only require that Stringify's demands be met?
> >
> > My thought would be that once you have an object in hand, you can do
> > with it what you will. But all you get is the object.
> How would you handle the following:
>    role Dog {must bark();}
>    role Tree {must bark();}
>    class crossPerson {
>       method bark() {speak_sharply;}
>    }
>    class Trog does Tree does Dog {
>       method bark() {bark_like_a_trog;}
>    }
>    multi sub purchase(Dog $mansBestFriend) {...}
>    multi sub purchase(Tree $shrubbery) {...}
>    multi sub purchase($noisemaker must bark()) {...}
>    my crossPerson $jack;
>    purchase $jack;
>    my Trog $spot;
>    purchase $spot;
> Which, if any, of the subs should be called in each case?  Or should the
> compiler complain of duplicate definitions?

$jack is a crossPerson, which absolutely does NOT have the Dog or Trog or
Tree classes in its C<isa> chain. If Dog and Tree are both "inferred" or
"inferrable" classes, then the two multi declarations (Dog and Tree
arguments) are identical.

Thus, there's probably a warning or an error about conflict of inference for
any call that's not passed an explicitly typed object.

Given the strictness of the C<role> behavior, I think that the consistent
answer is to fail as soon as possible and report that the three purchase
subs are in conflict with each other.

(In the generally interesting case of what to do with the Dog/Tree
multisubs, I think the right answer is to catch this either when class Trog
is declared, or when C<purchase $spot;> is called. Comments from @Larry,
especially Damian, would be nice.)


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