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Subject: Re: [PHP-DEV] RFC - Array slice syntactic sugar
From: rowan.collins@gmail.com (Rowan Collins)

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Alex Bowers wrote on 20/03/2015 11:38:
>
>     The @ symbol in my examples is not a special marker that is
>     meaningful on its own; the parser wouldn't even see it as a
>     separate token. The syntax for key access is $array[$key], the
>     syntax for positional access would be $array[@$position]; chosen
>     to look similar, but one is not a special case of the other.
>
>
> Ah, I understand you now. Yes I agree that this would be useful then. 
> It allows us the future possibility of a separate RFC looking into 
> slicing on keys as mentioned at the bottom of your post.
>
>     The syntax needn't be just an extra character, we could invent
>     something else, like $array[[$position]], or $array@($position),
>     though I can't think of anything I like.
>
>
> I think for simplicity, we should keep it as $array[], and have a 
> symbol within it that shows that it is an internal index rather than 
> the key. An Asterisk (*) is my preference at the moment, though caret 
> also looks promising.
>
> Here are some examples of how various characters could look
>
> $array[*1:3], $array[*:], $array[*:1], $array[*1:]
> $array[@1:3], $array[@:], $array[@:1], $array[@1:]
> $array[^1:3], $array[^:], $array[^:1], $array[^1:]
> $array[&1:3], $array[&:], $array[&:1], $array[&1:]
>
>     The symbol isn't something that can be missing, it's just part of
>     the syntax, so this is like saying "what should we do if the
>     closing ] is missing?" The answer is, nothing, it's undefined
>     syntax, so it's a parse error.
>
>
> So a parse error currently, and a future RFC can define the terms of 
> using this normally?
>
> If we go down the route of adding a symbol for by index rather than by 
> key, then this RFC should also include the normal getting array by key 
> style, without the colon range operator.
>
> For example:
>
> $array[*1] to get the item that is the second in the list, rather than 
> at key 1.

Yep, I think we're on the same page now.

There's basically three ways of selecting items - by key, by position 
from start, and by position from end - and two types of selection - a 
single element, or an array of zero or more elements. That gives us 6 
possible combinations, which (using the @ notation for consistency with 
previous examples) would be:

Select by key:
Single element: $array[$x]  - existing syntax
Slice: $array[$a:$b] - equivalent to a filter asserting $key >= $a && 
$key <= $b

Select by position from start:
Single element: $array[@$x]
Slice: $array[@$a:$b],$array[@$a:], and $array[@:$b]

Select by position from end - as above, but using negative indexes:
Single element: $array[@-$x] - equivalent to $array[@length($array)-$x]
Slice: $array[@-$a:-$b] - equivalent to 
$array[@length($array)-$a:length($array)-$b]


Several of these are actually rather hard to do with current PHP features:
- you can get the first element with reset() and the last with end(), 
but arbitrary position selection is harder
- there is no version of array_filter for testing keys, and being able 
to write $dictionary['elephant':'snake'] would actually be pretty handy

Actually, the only one that's easy to emulate (apart from the obvious 
single element by key) is the one you propose to add first - getting a 
positional slice of an array, which array_slice(..., true) already does.

Regards,
-- 
Rowan Collins
[IMSoP]

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