Hi all,
The named parameters RFC has been accepted, despite significant objections
from maintainers of larger OSS projects due to the overhead it adds to
maintaining backwards compatibility as it has now made method/function
parameter names part of the API; a change to them would cause a BC break
for any library users who decide to use the new feature.
It is likely that the way this will shake out is that some maintainers will
accept the additional overhead of including parameter names in their BC
guidelines and others will not, this leaves users unsure if they can use
the new feature without storing up issues in potentially minor/security
releases of the libraries they use. This is not really an ideal situation.
More pressing a point is that the current implementation breaks object
polymorphism. Consider this example (simplified from one of my codebases)
interface Handler {
public function handle($message);
}
class RegistrationHandler implements Handler {
public function handle($registraionCommand);
}
class ForgottenPasswordHandler implements Handler {
public function handle($forgottenPasswordCommand);
}
class MessageBus {
//...
public function addHandler(string $message, Handler $handler) { //... }
public function getHandler(string $messageType): Handler { //... }
public function dispatch($message)
{
$this->getHandler(get_class($message))->handle(message: $message);
}
}
This code breaks at run time.
Proposals were made for resolutions to this issue however all of them
require trade offs and could potentially break existing code.
My proposal to resolve these two issues is to add the ability to rename
parameters with a new syntax as follows.
function callBar(Foo $internalName:externalName) {
$internalName->bar();
}
$x = new Foo();
callBar(externalName: $x);
This allows both the above problems to be resolved, by renaming the
internal parameter and keeping the external signature the same.
I propose that the RFC would have two voting options.
The first would be to implement it as proposed above, this would allow any
parameter to be called by name regardless of the intentions of the author
of the method/function and is closest to the current behaviour.
The second option would be to use this syntax to make named parameters in
userland code explicitly opt in. As such an additional shortcut syntax
would be implemented: $: to designate a named parameter. eg
function callBar($:externalName) {
$externalName->bar();
}
$x = new Foo();
callBar(externalName: $x);
If a parameter is not opted in, a compile time error is raised:
function callBar($externalName) {
$externalName->bar();
}
$x = new Foo();
callBar(externalName: $x); // Error: cannot call parameter $externalName by
name.
There are pros and cons to this second approach, on the one hand it reduces
the usefulness of the named parameter syntax by requiring changes to old
code to enable it (although this could probably be automated fairly easily)
however it does provide a neater solution to the second problem in that, to
prevent the runtime errors in the second issue example, every child class
would need to use the rename syntax on it's parameter to prevent errors,
whereas if we went down this route, the parent class could just not opt
into the named parameter syntax and the code would function as expected.
Another advantage is that with the ability to rename parameters using the
opt in, we gain some flexibility to tighten up the LSP rules relating to
named parameter inheritance.
class Foo {
public function bar($:param) { //... }
public function baz($internal:external) { //... }
}
// OK
class Bar {
public function bar($renamed:param) { //... }
public function baz($renamed:external) { //... }
}
// Compile time error cannot rename named parameter $:param (renamed to
$:renamedParam)
class Baz {
public function bar($:renamedParam) { //... }
}
// Compile time error cannot rename named parameter $:external (renamed to
$:renamed)
class Baz {
public function baz($internal:renamed) { //... }
}
While this would be technically possible with the first option (no opt in)
it would break any existing code which renames a parameter as every
parameter would be subject to these rules.
I don't have Wiki karma so can't post this yet; but I want to get the ball
rolling on discussion as feature freeze is coming up fast and if we want to
go for the second option, that must hit before the named parameter syntax
is in a tagged version of PHP.
Regards,
Chris
