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Date: Wed, 12 May 2021 11:51:00 +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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> > > > 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?
>
> I'd have to defer to Joe (he wrote the implementation), but I suspect
> not.  The partial has to be there to carry around the information of what
> callable to actually call and what the bound parameters are.  At that
> point, it's likely more work to decompose the Partial internally before
> calling it than to just call the Partial itself.
>

Then I suppose that removing the extra frame would mean embedding that
state into Closure objects. Would it make sense to extend Closure this way?
Isn't the new Partial class an implementation artefact that should be
removed? If not, what are the userland-side reasons to keep it?

If the Partial class has to be kept, can it be optimized out back to a
closure when no arguments are bound? (aka the "$closure = $this->method(?)"
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
>
> I cannot think of a use case where that would be needed.  Since you can
> partial-ize any callable, including a dynamic one, if you needed to do
> something like partial-ize one of a series of function calls you can do
> that already:
>
> $c = match($some_input) {
>   'A' => 'func_a',
>   'B' => 'func_b',
>   'C' => 'func_c',
> };
>
> $p = $c(1, 2 ?, 4);
>
> Though at that point, just partialing them in the first place inside the
> match would be better as then you never have a function name in a string to
> begin with.
>

Here is a use case: high-order argument resolvers / function reducers.

What I mean is a function that takes a callable as arguments, resolves as
many args of the callable as it can using whatever logic fits, and returns
a callable with fewer arguments (only the non-resolved ones - aka a
Partial).

Nicolas

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