Type hints in array destructuring expressions

Hi Enno,

czw., 16 kwi 2020 o 12:20 Enno Woortmann enno.woortmann@web.de napisał(a):

Hi together,

as the voting for the "Type casting in array destructuring expressions"
shows a clear direction to be declined (sad faces on my side, I really
would've liked it as a feature completion of the casting feature set
without the need for a really new syntax, as the parser also already
coverred it, but ok, time to continue :D ), combined with a strong
interest for the topic "type checks in array destructuring expressions"
(as well in the discussion as in the vote) I've set up a first draft of
an RFC covering this topic:

https://wiki.php.net/rfc/typehint_array_desctructuring

The discussion around the casting in array destructuring brought up the
idea to perform regular type checks in array destructuring expressions.
This RFC proposes a new syntax to type hint a variable inside an array
destructuring expression:

$data = [42, 'Example', 2002];
[int $id, string $data, int $year] = $data;

I fear that if you go that path people would automatically expect it to be
possible to write:

int $id = $data['id'];
int $id = getIdFromData($data);

And then expect the $id to behave more like typed property with type guard.
At least this is me what would expect from putting a type in from of
variable name
like in array destructuring example.

Why? Cause it has a type constraint in from of variable name just like
typed properties have.

Cheers,
Michał Brzuchalski

Hi together,

as the voting for the "Type casting in array destructuring expressions"
shows a clear direction to be declined (sad faces on my side, I really
would've liked it as a feature completion of the casting feature set
without the need for a really new syntax, as the parser also already
coverred it, but ok, time to continue :D ), combined with a strong
interest for the topic "type checks in array destructuring expressions"
(as well in the discussion as in the vote) I've set up a first draft of
an RFC covering this topic:

https://wiki.php.net/rfc/typehint_array_desctructuring

The discussion around the casting in array destructuring brought up the
idea to perform regular type checks in array destructuring expressions.
This RFC proposes a new syntax to type hint a variable inside an array
destructuring expression:

$data = [42, 'Example', 2002];
[int $id, string $data, int $year] = $data;

The type hints behave identically to the type hints used in method
signatures (compare https://wiki.php.net/rfc/scalar_type_hints_v5). This
especially includes a behaviour depending on the strict_types directive.

Additionally to scalar type hints also object type hints are possible.

I've not yet started an implementation draft but I think this one will
be more tricky than the type cast implementation approach. As the parser
re-uses the array definition for array destructuring expressions (which
also leads to the type cast implementation without touching the parser)
adding type hints to the definition would also affect other parts of the
language eg. something like:

$x = [int $a, string $b];

would be parsed by the parser. I'm currently not able to grasp the
impact of such a change. But that's another topic.

Let's focus on the idea itself instead of the implementation first.
Thoughts on the first draft?

Cheers, Enno

--

Hi, thanks for proposing,

I guess it could be useful (or convenient at least). Just 2 thoughts:

• It has been requested several times on this list to stop using the
  wording "type hint" and instead write "type declaration" ;)

• When I see this example in the RFC:

$years = [["now", 2020], ["future", 2021]];
foreach ($years as [string $description, int $year]) {

I immediately think of a similar:

$years = ["now" => 2020, "future" => 2021];
foreach ($years as string $description => int $year) {

or:

$foos = [new Foo("one"), new Foo("two")];
foreach ($foos as Foo $foo) {

Now I remember this has already been requested (e.g.
https://externals.io/message/104485#104488), but that's not really the
same use case, so I don't know if it could/should be included...

Regards,

--
Guilliam Xavier

On Thu, Apr 16, 2020 at 12:20 PM Enno Woortmann enno.woortmann@web.de
wrote:

Hi together,

as the voting for the "Type casting in array destructuring expressions"
shows a clear direction to be declined (sad faces on my side, I really
would've liked it as a feature completion of the casting feature set
without the need for a really new syntax, as the parser also already
coverred it, but ok, time to continue :D ), combined with a strong
interest for the topic "type checks in array destructuring expressions"
(as well in the discussion as in the vote) I've set up a first draft of
an RFC covering this topic:

https://wiki.php.net/rfc/typehint_array_desctructuring

The discussion around the casting in array destructuring brought up the
idea to perform regular type checks in array destructuring expressions.
This RFC proposes a new syntax to type hint a variable inside an array
destructuring expression:

$data = [42, 'Example', 2002];
[int $id, string $data, int $year] = $data;

The type hints behave identically to the type hints used in method
signatures (compare https://wiki.php.net/rfc/scalar_type_hints_v5). This
especially includes a behaviour depending on the strict_types directive.

Additionally to scalar type hints also object type hints are possible.

I've not yet started an implementation draft but I think this one will
be more tricky than the type cast implementation approach. As the parser
re-uses the array definition for array destructuring expressions (which
also leads to the type cast implementation without touching the parser)
adding type hints to the definition would also affect other parts of the
language eg. something like:

$x = [int $a, string $b];

would be parsed by the parser. I'm currently not able to grasp the
impact of such a change. But that's another topic.

Let's focus on the idea itself instead of the implementation first.
Thoughts on the first draft?

As you say, this syntax will likely run into parsing issues. Once you take
into account that types aren't just "int", but also "Foo|Bar", and also
consider that we have support for references in unpacking, you could be
left with something like

[int|float &$foo] = $bar;

Without actually testing, I would expect that this is going to run into
issues of similar difficulty as the arrow function proposal encountered.

Rather than integrating this into the destructuring assignment syntax, I
think it would be more helpful to view this as the infallible variant of
pattern matching, which is also being discussed in the "match expression"
thread. As suggested there, the syntax could be something like

let [int|float $foo] = $bar;

and would throw if the RHS does not satisfy the pattern on the LHS (unlike
the match variant, which is fallible).

I think this might help to bring things under one umbrella, and to draw a
clearer line between this functionality and typed variables (if it's just a
pattern guard, then it's expected that the type restriction is checked
instantaneous, not enforced persistently.)

Nikita