PHP RFC: Pattern Matching

• Version: 0.9
• Date: 2020-11-11
• Author: Larry Garfield (larry@garfieldtech.com), Ilija Tovilo (tovilo.ilija@gmail.com)
• Status: Draft
• First Published at: http://wiki.php.net/rfc/pattern-matching

This RFC introduces the beginning of a pattern matching syntax for PHP. It does not include complete matching of all possible pattern types in order to keep the initial implementation simple and reduce bikeshedding, but does lay out the mechanism by which pattern matching operates. The Future Scope section includes recommendations for continued improvement in future RFCs.

Pattern Matching as a language concept contains two parts: Matching a variable against a potentially complex data structure pattern, and optionally extracting values out of that variable into their own variables. In a sense it serves a similar purpose for complex data structures as regular expressions do for strings. When properly applied, it can lead to very compact but still readable code, especially when combined with conditional structures such as match().

Pattern matching is found in a number of languages, including Haskell, C#, ML, Rust, and Swift, among others. The syntax offered here is inspired primarily by C#, but is not intended as as direct port.

This RFC is part of the Enumerations Epic. It is a stepping stone toward full Enumerations but stands on its own as useful functionality.

This RFC introduces a new keyword and binary operator: is. The is keyword indicates that its right hand side is a pattern against which its left hand side should be applied. The is operator is technically a comparison operator, and always returns a boolean true or false.

if($var is <pattern>) {

}

The left-hand side of is will be evaluated first until it is reduced to a single value (which could be an arbitrarily complex object or array). That value will then be compared to the pattern, and true or false returned.

While patterns may resemble other language constructs, whatever follows is is a pattern, not some other instruction.

is may be used in any context in which a boolean result is permissible. That includes variable assignment, if conditions, while conditions, match() statements, etc.

A pattern may be a type signature, including both class and primitive types as well as compound types. In this case, is will match the left hand side value against the specified type. That is, the following are all legal:

$foo is string;    // Equivalent to is_string($foo)
$foo is int|float; // Equivalent to is_int($foo) || is_float($foo)
$foo is Request;   // Equivalent to $foo instanceof Request
$foo is User|int;  // Equivalent to $foo instanceof User || is_int($foo)
$foo is ?array;    // Equivalent to is_array($foo) || is_null($foo)

A type match may be any syntax supported by a parameter type; in a sense, $foo is pattern is equivalent to “would $foo pass a type check if passed to a parameter with this type specification.” As more complex type checks become allowed (such as intersection types, type aliases, etc.) they will become valid in a pattern as well.

Type patterns respect the strict/weak typing mode of the file in which it is evaluated.

Any literal may be a pattern. This is a degenerate case and not generally useful, but is included for consistency when used with match() (see below).

$foo is 5;         // Equivalent to $foo === 5
$foo is 'yay PHP'; // Equivalent to $foo === 'yay PHP'

Global constants are NOT permitted in a pattern. They cannot be disambiguated from a class name, and are of minimal if any use in practice.

A pattern may also define an class and matches against scope-accessible properties of that object. Only a single class type may be used, but any number of properties may be matched. The properties must be accessible in the scope in which the pattern executes. That is, a pattern evaluated outside the class may only match against public properties; a pattern inside the class may match against public, private, or protected; a pattern in a child class may match against protected properties of its parent but not private; etc.

class Point {
  public function __construct(public int $x, public int $y, public int $z) {}
}
 
$p = new Point(3, 4, 5);
 
$p is Point {x: 3};
// Equivalent to:
$p instanceof Point && $p->x === 3;
 
$p is Point {y: 37, x: 2,};
// Equivalent to:
$p instanceof Point && $p->y === 37 && $p->x === 2;

Properties may be listed in any order. A trailing comma is permitted.

Pattern matching is frequently used in conjunction with branching structures, in particular with enumerations. To that end, this RFC also enhances the match() structure. Specifically, if the is keyword is used in match() then match() will perform a pattern match rather than an identity comparison.

That is, this code:

$result = match ($somevar) is {
    Foo => 'foo',
    Bar => 'bar',
    Baz|Beep => 'baz',
};

is equivalent to the following:

$result = match (true) {
    $somevar is Foo => 'foo',
    $somevar is Bar => 'bar',
    $somevar is Baz|Beep => 'baz',
};

One of the prime uses of pattern matching is to extract a value from a larger structure, such as an object (or Enumeration/ADT, in the future). This RFC supports such variable binding. A variable that should be bound is denoted by a % and a variable. If the input variable matches the rest of the pattern, then the corresponding value will be extracted and assigned to a variable of that name in the current scope. It will remain in scope as long as normal variable rules say it should.

In the currently supported patterns, it is only relevant for object pattern matching.

class Point {
  public function __construct(public int $x, public int $y, public int $z) {}
}
 
$p = new Point(3, 4, 5);
 
if ($p is Point {x: 3, y: %$y} ) {
  print "x is 3 and y is $y.";
}
// Equivalent to:
if ($p instanceof Point && $p->x === 3) {
  $y = $p->y;
  print "x is 3 and y is $y.";
}
 
if ($p is Point {z: %$z, x: 3, y: %$y} ) {
  print "x is 3 and y is $y and z is $z.";
}
// Equivalent to:
if ($p instanceof Point && $p->x === 3) {
  $y = $p->y;
  $z = $p->z;
  print "x is 3 and y is $y and z is $z.";
}

If the variable name to extract to is the same as the name of the property, then the property name may be omitted. That is, the last example can be abbreviated as:

if ($p is Point {%$z, x: 3, %$y} ) {
  print "x is 3 and y is $y and z is $z.";
}

Variable binding is especially useful in match() statements, where there is no simple logical equivalent that doesn't involve additional functions.

$result = match ($p) is {
  // These will match only some Point objects, depending on their property values.
  Point{x: 3, y: 9, %$z} => "x is 3, y is 9, z is $z",
  Point{%$z, %$x, y: 4} => "x is $x, y is 4, z is $z",
  Point{x: 5, %$y} => "x is 5, y is $y, and z doesn't matter",
  // This will match any Point object.
  Point{%$x, %$y, %$z} => "x is $x, y is $y, z is $z",
};

Note that in this case, the variables $x, $y, and $z may or may not be defined after the match() statement executes depending on which pattern was matched.

A new keyword is added, is. That conflicts with a global constant named is.

No other BC breaks are expected.

PHP 8.next (aka 8.1).

Do any other patterns need to be included in the initial RFC?

The % flag for binding is still an open question. It would be necessary if we want to allow variables to be used in the pattern, but so far we haven't decided if variables belong in the pattern. Open question for discussion.

Numerous other, more robust (and complex) patterns can be supported in the future. This RFC keeps to the MVP implementation and most common cases. The following additional patterns are possible future additions for other RFCs. (Please don't bikeshed them here; they are shown as an example of where pattern matching can extend to in the future.)

$arr is ['a' => 'A', 'b' => $b];
 
// Equivalent to:
is_array($arr) && $arr['a'] === 'A' && $arr['b'] === $b);

$foo is 0..=10;
 
// Equivalent to:
$foo >=0 && $anInt <= 10;
 
$foo is 0..<10;
 
// Equivalent to:
$foo >=0 && $anInt < 10;
 
$foo is >10;
 
// Equivalent to:
$foo > 10;

$foo is 1 or 2;
 
// Equivalent to:
$foo === 1 || $foo === 2;
 
$foo is User or 1..=5;
 
// Equivalent to:
$foo instanceof User || ($foo >= 0 && $foo <= 5);

$foo is /^http:\/\/%$domain/
 
// Equivalent to:
$matches = [];
preg_match('/^http:\/\/%$domain/', $foo, $matches);
$domain == $matches[0];

This is a simple up-or-down vote, requiring 2/3 Yes to pass.

Links to any external patches and tests go here.

If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.

Make it clear if the patch is intended to be the final patch, or is just a prototype.

For changes affecting the core language, you should also provide a patch for the language specification.

After the project is implemented, this section should contain

1. the version(s) it was merged into
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature
4. a link to the language specification section (if any)

Links to external references, discussions or RFCs

Keep this updated with features that were discussed on the mail lists.