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Date: Wed, 25 Mar 2020 13:10:05 +0100
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Thread-Topic: [RFC] switch expression
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Subject: [RFC] switch expression
From: ilija.tovilo@me.com (Ilija Tovilo)

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Hi everybody!

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A few years ago I suggested adding a new `match` expression to the PHP lang=
uage:

https://externals.io/message/100487

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I arrogantly assumed someone will implement it for me which of course didn'=
t happen. I'd finally like to get my own hands dirty. I have a very rough, i=
ncomplete prototype but I'd like to get your feedback before I continue work=
ing on the details.

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# Introduction

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This is what it looks like:

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```php

echo $i switch {

=C2=A0=C2=A0=C2=A0 0 =3D> "i equals 0",

=C2=A0=C2=A0=C2=A0 1 =3D> "i equals 1",

=C2=A0=C2=A0=C2=A0 2 =3D> "i equals 2",

=C2=A0=C2=A0=C2=A0 3, 4 =3D> "i equals 3 or 4",

};

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// is roughly equivalent to

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switch ($i) {

=C2=A0=C2=A0=C2=A0 case 0:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $tmp =3D "i equals 0";

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =C2=A0break;

=C2=A0=C2=A0=C2=A0 case 1:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $tmp =3D "i equals 1";

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 break;

=C2=A0=C2=A0=C2=A0 case 2:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $tmp =3D "i equals 2";

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 break;

=C2=A0=C2=A0=C2=A0 case 3:

=C2=A0=C2=A0=C2=A0 case 4:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $tmp =3D "i equals 3 or 4";

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 break;

=C2=A0=C2=A0=C2=A0 default:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 throw new InvalidArgumentException('Unhandled switch case');

}

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echo $tmp;

```

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Some things to note:

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* Each case only accepts a single expression

* The entire switch expression evaluates to the result of the executed case

* There is no fallthrough, an implicit break is added after every case

* Multiple case conditions are possible with comma separation

* The default case throws a InvalidArgumentException by default

* The switch keyword is used as an infix operator

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# Syntax

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Originally, I expected to reuse the current syntax and transform it into an=
 expression.

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```php

$x =3D switch ($y) { ... };

```

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Turns out this is ambiguous.

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```php

switch ($y) { ... }

[$a] =3D ...;

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// Could also be interpreted as

switch ($y) { ... }[$a] =3D ...;

```

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I stole the new syntax from C# 8.0 which means at least some people will al=
ready be familiar with it:

https://docs.microsoft.com/en-us/archive/msdn-magazine/2019/may/csharp-8-0-=
pattern-matching-in-csharp-8-0#the-evolution-of-pattern-matching-in-c-80

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# Type coercion

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One of the bigger weak points of the `switch` statement is the fact that it=
 performs implicit type coercion.

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```php

switch ('foo') {

=C2=A0=C2=A0=C2=A0 case 0:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 echo "Oh no!\n";

}

```

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While it's very tempting to fix this in the new `switch` expression it adds=
 a confusing discrepancy between the `switch` statement and expression. I th=
ink it would be preferrable to keep the two the same and change the behavior=
 of both in a new PHP edition (https://github.com/php/php-rfcs/pull/2).

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# Pattern matching

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I decided against pattern matching because PHP doesn't have algebraic data =
types and classes rarely have public properties. In my opinion the limited u=
se cases don't justify the significant complexity added to the language. It =
would also, once again, add an unjustified discrepancy between the `switch` =
statement and expression. If at some point we do want to introduce pattern m=
atching it might be better to introduce a different keyword (e.g. `match`) a=
nd make it work for both the statement and expression. In case you need to m=
atch a more complex expression the following still works fine:

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```php

echo true switch {

=C2=A0=C2=A0=C2=A0 is_int($x) =3D> 'int',

=C2=A0=C2=A0=C2=A0 is_float($x) =3D> 'float',

=C2=A0=C2=A0=C2=A0 is_string($x) =3D> 'string',

=C2=A0=C2=A0=C2=A0 ...

};

```

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# Blocks

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Sometimes it would be useful to split the expression into multiple statemen=
ts to make it more readable. Unfortunately, in PHP there are no block expres=
sions. Rust allows returning the last value by omitting the semicolon:

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```php

echo $x switch {

=C2=A0=C2=A0=C2=A0 1 =3D> {

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 foo();

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bar();

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 baz()

=C2=A0=C2=A0=C2=A0 },

};

```

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This is indeed possible in PHP and could be implemented as part of the `swi=
tch` expression or as a general language feature. A nice side effect is that=
 this could also be used in arrow functions:

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```php

$x =3D fn() =3D> {

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 foo();

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bar();

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 baz()

};

```

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This would, however, make it inconsistent with closures as they use the ret=
urn keyword. Thus we would probably have to make sure arrow functions still =
work with return statement which would decrease the need for such a language=
 construct. It is also very unlike anything else in PHP.

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# Poll

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This is a short overview of what I'll be working on in the coming weeks. I =
created a short poll for you guys to let me know if this idea is worth pursu=
ing:

https://forms.gle/stXMv72CAaDDxfwf8

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Stay safe!

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