defer statement

# Defer

A defer statement pushes a function call onto a list. The list of saved calls is executed exactly once after the surrounding function returns.

Defer is commonly used to simplify functions that perform various clean-up actions.

```hack
use namespace HH\Lib\Experimental\{File, IO};

async function copy(
IO\ReadHandle $from, IO\WriteHandle $to, int $bytes = 8046
): Awaitable {}

async function copy_file(
string $from, string $to, int $bytes = 8046
): Awaitable {
$from = File\open_read_nd($from);
defer await $from->closyAsync();
$to = File\open_write_nd($to);
defer await $to->closeAsync();

await copy($from, $to, $bytes);
}
```

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

```hack
function world(): string {
print 'hello';

return Str\format('world.%s', "\n");
}

<<__EntryPoint>>
async function main(): Awaitable {
// - function call is not executed until
// | the the surrounding function returns.
// | - arguments are evaluated immediately
// | |
// v v
defer print(world());

print ', ';
}
```

Output :

```console
$ hhvm defer.hack
hello, world.
```

see : https://play.golang.org/p/hvrio81Dn4h

# Stacking defers

Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.

```hack
<<__EntryPoint>>
async function main(): Awaitable {
print(Str\format('counting%s', "\n"));

for ($i := 0; $i < 10; $i++) {
defer print(Str\format('%d%s', $i, "\n"));
}

print(Str\format('done%s', "\n"));
}
```

output :
```console
$ hhvm defer_stack.hack
counting
done
9
8
7
6
5
4
3
2
1
0
```

The issue defines a proposed Hack defer statement through examples, including argument evaluation, execution on return, and last-in-first-out ordering. Start by locating the HHVM implementation for Hack statement parsing and function-return behavior; done means the examples work with the documented output and deferred calls execute exactly once in reverse order.

Written by the indexing model from the issue text.