[RFC] Switch expression

[iluuu1994]

Implementation for the coming switch expression RFC.

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

[ostrolucky]

Hello, can you explain what's difference between

return $x switch {
        0 => 'Zero',
        1 => 'One',
        2 => 'Two',
        3 => 'Three',
        4 => 'Four',
        5 => 'Five',
        6 => 'Six',
        7 => 'Seven',
        8 => 'Eight',
        9 => 'Nine',
    };

and

return [
        0 => 'Zero',
        1 => 'One',
        2 => 'Two',
        3 => 'Three',
        4 => 'Four',
        5 => 'Five',
        6 => 'Six',
        7 => 'Seven',
        8 => 'Eight',
        9 => 'Nine',
][$x];

? Because currently I don't see advantage of your proposal over simple array map.

[iluuu1994]

Your example will execute every arm.

return [
    0 => expensiveFunction1(),
    1 => expensiveFunction2(),
    ...
][$x];

Furthermore it builds a hash map in memory every time it executes. That is of course given the array isn't constant.

[ostrolucky]

Indeed. So it's more like

return [
        0 => fn() => 'Zero',
        1 => fn() => 'One',
        2 => fn() => 'Two',
        ...
][$x]();

[brzuchal]

I prefer the return switch (expr) {}; form for three reasons:

• it looks similar to switch statement we already have;
• it doesn't look like the $x variable is gonna be populated with the result from switch expression;
• it doesn't confuse like here print !true switch {}; where you have to think if ! has higher precedence or not.

---

I'm ok with losing the case keyword but am not sure if termination of a case by ; would be a better choice.
About the switch(true) expression, the simple return switch {}; I think could do the job and still be much readable. If there is a need for false switch expression then it can be put into parentheses - personally I think it would be a minor case.

[iluuu1994]

@brzuchal

I prefer the return switch (expr) {}; form for three reasons:

I agree but as mentioned the syntax is ambiguous. Context-sensitive grammar has not been well accepted in PHP.

About the switch(true) expression, the simple return switch {};

That's a nice idea 🙂

[iluuu1994]

@ostrolucky

Almost, you're still missing the default case.

It is in fact equivalent to:

switch ($x) {
    case 0:
        $tmp = 'Zero',
        break;
    case 1:
        $tmp = 'One',
        break;
    ...
    default:
        throw new InvalidArgumentException();
}

return $tmp;

It reuses the existing switch AST, code generation, opcodes, etc.

[iluuu1994]

We could actually probably reuse the ZEND_AST_EXPR_LIST.

[iluuu1994]

@webmozart I found a way to resolve the ambiguity after all. The only downside is that empty switch expressions are disallowed:

// This throws a parser error
$x => switch ($y) {};

[webmozart]

@iluuu1994 Nice! IMHO there's no valid use case for that, so I would expect a parser error to be thrown.

[iluuu1994]

This test is useless now.

[iluuu1994]

This syntax needs to be updated.

[iluuu1994]

Superseded by #5371.