autocapture-rfc-wants.txt

Here are three things I'd like to see in any new auto-capturing closures RFC:


1. JUSTIFICATION

Don't assume that everyone already thinks it's necessary and desirable.
Try to come up with some real-life examples where you would want to use it.
Acknowledge the possible down-sides, and explain why you think they're
justified.

Be aware that the current set of features is not an accident, it's what was
agreed on in previous discussions. In particular, see this summary of the 
series of RFCs that led up to the "fn() => expression" feature: 
https://externals.io/message/113740#113833


2. SYNTAX

I am not at all convinced that "auto-capture" and "short" should go
together. We could make "fn" a synonym for "function" independent of
auto-capturing, so let's consider the syntax more carefully.

Do we want auto-capture to be considered the default, or an opt-in 
for certain use cases, and how can the syntax reflect that?
Do we want to allow a mixture of auto-capture and by-reference capture,
and what would that look like?

My preference remains for "use(*)" as a visible opt in to auto-capture,
with "use(*, &$foo)" to mean "$foo by reference, everything else needed
by value".


3. SEMANTICS

Unlike most languages, PHP lacks any keyword or syntax to declare a 
local variable, because variables are local by default. That means
we need to define exactly which variables mentioned in a closure
should be captured, and which should be local to the closure.

Even though capture is by value, capturing a variable unexpectedly
has an impact on object and resource lifetime (e.g. closing a network
connection in a destructor), and could leak memory for large values
(e.g. arrays and strings).

(This is technically true of the existing short closures, but it's
very rare to want a purely local variable inside a single expression.)

One option would be to introduce a var/let keyword for block scoping
first (as a separate RFC) and then auto-capture could mean "capture
everything that's not block scoped".


-- 
Rowan Tommins
[IMSoP]