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Date: Wed, 1 Feb 2017 23:06:04 +0000
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Subject: Re: [PHP-DEV] Re: [RFC][Discuss] Arrow Functions
From: rowan.collins@gmail.com (Rowan Collins)

On 01/02/2017 20:17, Rasmus Lerdorf wrote:
> You probably think of $var as being a local scope variable here,
> but really it isn't. It is an imported variable that happened to not exist
> in the outer scope and it was assigned a value in the inner scope, but that
> assignment can't leak back out so it is still unassigned in the outer
> scope.

I'm very confused by your terminology here. You definitely talk about 
two distinct scopes, "the inner scope" and "the outer scope", but then 
you assert that there is no "local scope". Surely the "inner scope" is 
"local" to the closure?

In actual fact, it's local to *each invocation* of the closure, because 
the function is initialised with the original values each time. For 
instance:

$a = 1;
$fn = fn() => ++$a;
$a = 42;
echo $fn(), ' ', $fn, ' ', $fn();

This echoes 2 2 2, not 2 3 4, because the $a being incremented is new 
for each invocation. Nor does it echo 43 43 43, because it is only the 
value of $a that is captured, not its reference.

I think the best description is this: the closure has its own scope, 
which is initialised on each invocation with a copy of another scope 
taken when it was defined.


To go back to Bruce's question, the original statement was:

> you don't typically need a local scope at all for these short closures

The important point here is that within a single expression there is 
very little you can meaningfully do with a local scope other than read 
from it. It's generally a bad idea to alter the value of a variable and 
expect another part of the same expression to see the new value, because 
you can't always predict how the compiler will rewrite the expression (I 
think an example of this came up on the list a while back, but can't 
think how to search for it).

So for most purposes you can consider all the automatically imported 
variables to be immutable values, which makes their scope much less 
relevant.

Regards,

-- 
Rowan Collins
[IMSoP]