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Date: Tue, 11 May 2021 20:32:24 +0200
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To: Larry Garfield <larry@garfieldtech.com>
Cc: php internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] Partial function application
From: nicolas.grekas+php@gmail.com (Nicolas Grekas)

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> > > On Sun, Apr 25, 2021, at 2:25 PM, Larry Garfield wrote:
> > > > Greetings, Internalians!
> > > >
> > > > I would like to offer for your consideration another RFC,
> specifically
> > > > syntax for partial function application.
> > > >
> > > > https://wiki.php.net/rfc/partial_function_application
> > > >
> > > > It includes an implementation by Joe Watkins that is already about
> 95%
> > > > complete.  (There's some edge cases he's still sorting out, but all
> of
> > > > the typical cases should work already.)  Most of the design work
> comes
> > > > from Levi Morrison and Paul Crovella.  I helped out with the tests, a
> > > > few edge bits, and general instigator/nudge. :-)
> > > >
> > > > Discuss.
> > >
> > > It looks like the conversation has died down, and it's been two weeks,
> so
> > > pending any other notable feedback I'll open a vote on this RFC on
> Thursday
> > > or Friday.
> > >
> > > --Larry Garfield
> > >
> >
> > LGTM, thanks for the RFC!
> >
> > What about visibility? I suppose this works even outside the object
> scope?
> > should this be mentionned?
> > $foo = $this->somePrivateMethod(1, ?)
>
> We're pretty sure it will work, and if you return $foo to another caller
> and then call it, it will work fine.  We'll add a test to confirm that.
>
> > Would it make sense to support a way to use a placeholder for "all
> > remaining args"?
> > Eg:
> > $foo = some_func(1, ...?)
>
> That's already implicit in the way it works now.  If you placeholder at
> least one parameter, and then don't fill in all of the remaining
> parameters, any additional trailing parameters are always placeholdered.
> So $obj->foo(?) will always return a partial that has the same arity as
> foo() does, regardless of how many parameters it has.
>
> > Combined with my previous comment, this could replace
> > Closure::fromCallable() by a language construct, which is something that
> we
> > already discussed in another thread (we talked about some ::function
> > special suffix instead):
> > $foo = $this->somePrivateMethod(...?)
>
> Yep.  That's noted in the RFC.  Although not the primary intent, a nice
> side effect is that any_func(?) is now a safe way to turn any
> function/method into a callable to reference it.  That renders ::function
> or similar 98% unnecessary.  (The remaining cases are mostly where you need
> to collect data statically for compilation or similar.)
>

Great news!

> In this last form, we might make this result in Closure::fromCallable()
> > exactly. Aka no increase of the depth of the stack trace.
> >
> > BTW, ideally, partial functions should not increase the depth of the
> > stacktrace at all. Do they?
> >
> > Nicolas
>
> They currently do, since they work by creating a Closure-esque object
> called Partial with an __invoke() method.  However, if you partial the same
> thing multiple times then only one stack level gets added.
>

Nice. Would it be possible to optimize this and remove the extra frame? At
least maybe for the (?) case?

This makes me wonder: can we create a partial programmatically? Wouldn't
that be needed for some use cases?
Partial::createFromCallable($callable, the-args)?

Nicolas

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