Pattern matching

Overview
Pattern Syntax and Semantics
Pattern usage
- Pattern match control flow
  - Guards
- Pattern matching in local variables
Open questions
- Slice or array nested value pattern matching
- Pattern matching as function overload resolution
Alternatives considered
References

Overview

A pattern is an expression-like syntax that describes the structure of some value. The pattern may contain unknowns, so it can potentially match multiple values, and those unknowns may have names, in which case they are called binding patterns. When a pattern is executed by giving it a value called the scrutinee, it determines whether the scrutinee matches the pattern, and if so, determines the values of the bindings.

Pattern Syntax and Semantics

Expressions are patterns, as described below. A pattern that is not an expression, because it contains pattern-specific syntax such as a binding pattern, is a proper pattern. Many expression forms, such as arbitrary function calls, are not permitted as proper patterns, so cannot contain binding patterns.

pattern ::= proper-pattern

fn F(n: i32) -> i32 { return n; }

match (F(42)) {
// ❌ Error: binding can't appear in a function call.
case (F(n: i32)) => {}
}

Expression patterns

An expression is a pattern.

pattern ::= expression

The pattern is compared with the expression using the == operator: pattern == scrutinee.

fn F(n: i32) {
  match (n) {
    // ✅ Results in an `n == 5` comparison.
    // OK despite `n` and `5` having different types.
    case 5 => {}
  }
}

Any == operations performed by a pattern match occur in lexical order, but for repeated matches against the same pattern, later comparisons may be skipped by reusing the result from an earlier comparison:

class ChattyIntMatcher {
  impl as EqWith(i32) {
    fn Eq[me: ChattyIntMatcher](other: i32) {
      Print("Matching {0}", other);
      return other == 1;
    }
  }
}

fn F() {
  // Prints `Matching 1` then `Matching 2`,
  // may or may not then print `Matching 1` again.
  match ((1, 2)) {
    case ({} as ChattyIntMatcher, 0) => {}
    case (1, {} as ChattyIntMatcher) => {}
    case ({} as ChattyIntMatcher, 2) => {}
  }
}

Alternatives considered

Introducer syntax for expression patterns

Binding patterns

Name binding patterns

A name binding pattern is a pattern.

binding-pattern ::= identifier : expression
proper-pattern ::= binding-pattern

The identifier specifies the name of the binding. The type of the binding is specified by the expression. The scrutinee is implicitly converted to that type if necessary. The binding is then bound to the converted value.

fn F() -> i32 {
  match (5) {
    // ✅ `5` is implicitly converted to `i32`.
    case n: i32 => {
      // The binding `n` has the value `5 as i32`,
      // which is the value returned.
      return n;
    }
  }
}

When a new object needs to be created for the binding, the lifetime of the bound value matches the scope of the binding.

class NoisyDestructor {
  fn Make() -> Self { return {}; }
  impl i32 as ImplicitAs(NoisyDestructor) {
    fn Convert[me: i32]() -> Self { return Make(); }
  }
  destructor {
    Print("Destroyed!");
  }
}

fn G() {
  // Does not print "Destroyed!".
  let n: NoisyDestructor = NoisyDestructor.Make();
  Print("Body of G");
  // Prints "Destroyed!" here.
}

fn H(n: i32) {
  // Does not print "Destroyed!".
  let (v: NoisyDestructor, w: i32) = (n, n);
  Print("Body of H");
  // Prints "Destroyed!" here.
}

Unused bindings

A syntax like a binding but with _ in place of an identifier, or unused before the name, can be used to ignore part of a value. Names that are qualified with the unused keyword are visible for name lookup but uses are invalid, including when they cause ambiguous name lookup errors. If attempted to be used, a compiler error will be shown to the user, instructing them to either remove the unused qualifier or remove the use.

binding-pattern ::= _ : expression

binding-pattern ::= unused identifier : expression

fn F(n: i32) {
match (n) {
// ✅ Matches and discards the value of `n`.
case _: i32 => {}
// ❌ Error: unreachable.
default => {}
}
}

As specified in #1084, function redeclarations may replace binding names with _s but may not use different names.

fn G(n: i32);
fn H(n: i32);
fn J(n: i32);

// ✅ Does not use `n`.
fn G(_: i32) {}
// ❌ Error: name of parameter does not match declaration.
fn H(m: i32) {}
// ✅ Does not use `n`.
fn J(unused n: i32);

Alternatives considered

Compile-time bindings

A :! can be used in place of : for a binding that is usable at compile time.

compile-time-pattern ::= template? identifier :! expression
compile-time-pattern ::= template? _ :! expression
compile-time-pattern ::= unused template? identifier :! expression

proper-pattern ::= compile-time-pattern

// ✅ `F` takes a symbolic facet parameter `T` and a parameter `x` of type `T`.
fn F(T:! type, x: T) {
var v: T = x;
}

The template keyword indicates the binding pattern is introducing a template binding, so name lookups into the binding will not be fully resolved until its value is known.

`auto` and type deduction

The auto keyword is a placeholder for a unique deduced type.

expression ::= auto

fn F(n: i32) {
var v: auto = SomeComplicatedExpression(n);
// Equivalent to:
var w: T = SomeComplicatedExpression(n);
// ... where `T` is the type of the initializer.
}

The auto keyword is only permitted in specific contexts. Currently these are:

As the return type of a function.
As the type of a binding.

It is anticipated that auto may be permitted in more contexts in the future, for example as a placeholder argument in a parameterized type that appears in a context where auto is allowed, such as Vector(auto) or auto*.

When the type of a binding requires type deduction, the type is deduced against the type of the scrutinee and deduced values are substituted back into the type before pattern matching is performed.

fn G[T:! Type](p: T*);
class X { impl as ImplicitAs(i32*); }
// ✅ Deduces `T = i32` then implicitly and
// trivially converts `p` to `i32*`.
fn H1(p: i32*) { G(p); }
// ❌ Error, can't deduce `T*` from `X`.
fn H2(p: X) { G(p); }

The above is only an illustration; the behavior of type deduction is not yet specified.

Alternatives considered

Shorthand for auto

`var`

A var prefix indicates that a pattern provides mutable storage for the scrutinee.

proper-pattern ::= var proper-pattern

A var pattern matches when its nested pattern matches. The type of the storage is the resolved type of the nested pattern. Any binding patterns within the nested pattern refer to portions of the corresponding storage rather than to the scrutinee.

fn F(p: i32*);
fn G() {
  match ((1, 2)) {
    // `n` is a mutable `i32`.
    case (var n: i32, 1) => { F(&n); }
    // `n` and `m` are the elements of a mutable `(i32, i32)`.
    case var (n: i32, m: i32) => { F(if n then &n else &m); }
  }
}

Pattern matching precedes the initialization of the storage for any var patterns. An introduced variable is only initialized if the complete pattern matches.

class X {
  destructor { Print("Destroyed!"); }
}
fn F(x: X) {
  match ((x, 1 as i32)) {
    case (var y: X, 0) => {}
    case (var z: X, 1) => {}
    // Prints "Destroyed!" only once, when `z` is destroyed.
  }
}

A var pattern cannot be nested within another var pattern. The declaration syntax var pattern = expresson ; is equivalent to let var pattern = expression ;.

Tuple patterns

A tuple of patterns can be used as a pattern.

tuple-pattern ::= ( [expression ,]* proper-pattern [, pattern]* ,? )
proper-pattern ::= tuple-pattern

A tuple-pattern containing no commas is treated as grouping parens: the contained proper-pattern is matched directly against the scrutinee. Otherwise, the behavior is as follows.

A tuple pattern is matched left-to-right. The scrutinee is required to be of tuple type.

Note that a tuple pattern must contain at least one proper-pattern. Otherwise, it is a tuple-valued expression. However, a tuple pattern and a corresponding tuple-valued expression are matched in the same way because == for a tuple compares fields left-to-right.

Struct patterns

A struct can be matched with a struct pattern.

proper-pattern ::= { [field-init ,]* proper-field-pattern [, field-pattern]* }
proper-pattern ::= { [field-pattern ,]+ _ }
field-init ::= designator = expression
proper-field-pattern ::= designator = proper-pattern
proper-field-pattern ::= binding-pattern
field-pattern ::= field-init
field-pattern ::= proper-field-pattern

A struct pattern resembles a struct literal, with at least one field initialized with a proper pattern:

match ({.a = 1, .b = 2}) {
  // Struct literal as an expression pattern.
  case {.b = 2, .a = 1} => {}
  // Struct pattern.
  case {.b = n: i32, .a = m: i32} => {}
}

The scrutinee is required to be of struct type, and to have the same set of field names as the pattern. The pattern is matched left-to-right, meaning that matching is performed in the field order specified in the pattern, not in the field order of the scrutinee. This is consistent with the behavior of matching against a struct-valued expression, where the expression pattern becomes the left operand of the == and so determines the order in which == comparisons for fields are performed.

In the case where a field will be bound to an identifier with the same name, a shorthand syntax is available: a: T is synonymous with .a = a: T.

match ({.a = 1, .b = 2}) {
  case {a: i32, b: i32} => { return a + b; }
}

If some fields should be ignored when matching, a trailing , _ can be added to specify this:

match ({.a = 1, .b = 2}) {
  case {.a = 1, _} => { return 1; }
  case {b: i32, _} => { return b; }
}

This is valid even if all fields are actually named in the pattern.

Alternatives considered

Struct pattern syntax

Alternative patterns

An alternative pattern is used to match one alternative of a choice type.

proper-pattern ::= callee-expression tuple-pattern
proper-pattern ::= designator tuple-pattern?

Here, callee-expression is syntactically an expression that is valid as the callee in a function call expression, and an alternative pattern is syntactically a function call expression whose argument list contains at least one proper-pattern.

If a callee-expression is provided, it is required to name a choice type alternative that has a parameter list, and the scrutinee is implicitly converted to that choice type. Otherwise, the scrutinee is required to be of some choice type, and the designator is looked up in that type and is required to name an alternative with a parameter list if and only if a tuple-pattern is specified.

The pattern matches if the active alternative in the scrutinee is the specified alternative, and the arguments of the alternative match the given tuple pattern (if any).

choice Optional(T:! Type) {
  None,
  Some(T)
}

match (Optional(i32).None) {
  // ✅ `.None` resolved to `Optional(i32).None`.
  case .None => {}
  // ✅ `.Some` resolved to `Optional(i32).Some`.
  case .Some(n: i32) => { Print("{0}", n); }
  // ❌ Error, no such alternative exists.
  case .Other => {}
}

class X {
  impl as ImplicitAs(Optional(i32));
}

match ({} as X) {
  // ✅ OK, but expression pattern.
  case Optional(i32).None => {}
  // ✅ OK, implicitly converts to `Optional(i32)`.
  case Optional(i32).Some(n: i32) => { Print("{0}", n); }
}

Note that a pattern of the form Optional(T).None is an expression pattern and is compared using ==.

Templates

Any checking of the type of the scrutinee against the type of the pattern that cannot be performed because the type of the scrutinee involves a template parameter is deferred until the template parameter's value is known. During instantiation, patterns that are not meaningful due to a type error are instead treated as not matching. This includes cases where an == fails because of a missing EqWith implementation.

fn TypeName[template T:! Type](x: T) -> String {
  match (x) {
    // ✅ OK, the type of `x` is a template parameter.
    case _: i32 => { return "int"; }
    case _: bool => { return "bool"; }
    case _: auto* => { return "pointer"; }
    default => { return "unknown"; }
  }
}

Cases where the match is invalid for reasons not involving the template parameter are rejected when type-checking the template:

fn MeaninglessMatch[template T:! Type](x: T*) {
  match (*x) {
    // ✅ OK, `T` could be a tuple.
    case (_: auto, _: auto) => {}
    default => {}
  }
  match (x->y) {
    // ✅ OK, `T.y` could be a tuple.
    case (_: auto, _: auto) => {}
    default => {}
  }
  match (x) {
    // ❌ Error, tuple pattern cannot match value of non-tuple type `T*`.
    case (_: auto, _: auto) => {}
    default => {}
  }
}

Refutability, overlap, usefulness, and exhaustiveness

Some definitions:

A pattern P is useful in the context of a set of patterns C if there exists a value that P can match that no pattern in C matches.
A set of patterns C is exhaustive if it matches all possible values. Equivalently, C is exhaustive if the pattern _: auto is not useful in the context of C.
A pattern P is refutable if there are values that it does not match, that is, if the pattern _: auto is useful in the context of {P}. Equivalently, the pattern P is refutable if the set of patterns {P} is not exhaustive.
A set of patterns C is overlapping if there exists any value that is matched by more than one pattern in C.

For the purpose of these terms, expression patterns that match a constant tuple, struct, or choice value are treated as if they were tuple, struct, or alternative patterns, respectively, and bool is treated like a choice type. Any expression patterns that remain after applying this rule are considered to match a single value from an infinite set of values so that a set of expression patterns is never exhaustive:

fn IsEven(n: u8) -> bool {
  // Not considered exhaustive.
  match (n) {
    case 0 => { return true; }
    case 1 => { return false; }
    ...
    case 255 => { return false; }
  }
  // Code here is considered to be reachable.
}

fn IsTrue(b: bool) -> bool {
  // Considered exhaustive.
  match (b) {
    case false => { return false; }
    case true => { return true; }
  }
  // Code here is considered to be unreachable.
}

When determining whether a pattern is useful, no attempt is made to determine the value of any guards, and instead a worst-case assumption is made: a guard on that pattern is assumed to evaluate to true and a guard on any pattern in the context set is assumed to evaluate to false.

We will diagnose the following situations:

A pattern is not useful in the context of prior patterns. In a match statement, this happens if a pattern or default cannot match because all cases it could cover are handled by prior cases or a prior default. For example:

choice Optional(T:! Type) {
  None,
  Some(T)
}
fn F(a: Optional(i32), b: Optional(i32)) {
  match ((a, b)) {
    case (.Some(a: i32), _: auto) => {}
    // ✅ OK, but only matches values of the form `(None, Some)`,
    // because `(Some, Some)` is matched by the previous pattern.
    case (_: auto, .Some(b: i32)) => {}
    // ✅ OK, matches all remaining values.
    case (.None, .None) => {}
    // ❌ Error, this pattern never matches.
    case (_: auto, _: auto) => {}
  }
}

A pattern match is not exhaustive and the program doesn't explicitly say what to do when no pattern matches. For example:
- If the patterns in a match are not exhaustive and no default is provided.
```
fn F(n: i32) -> i32 {
  // ❌ Error, this `match` is not exhaustive.
  match (n) {
    case 0 => { return 2; }
    case 1 => { return 3; }
    case 2 => { return 5; }
    case 3 => { return 7; }
    case 4 => { return 11; }
  }
}
```
- If a refutable pattern appears in a context where only one pattern can be specified, such as a let or var declaration, and there is no fallback behavior. This currently includes all pattern matching contexts other than match statements, but the var/let-else feature in #1871 would introduce a second context permitting refutable matches, and overloaded functions might introduce a third context.
```
fn F(n: i32) {
  // ❌ Error, refutable expression pattern `5` used in context
  // requiring an irrefutable pattern.
  var 5 = n;
}
// ❌ Error, refutable expression pattern `5` used in context
// requiring an irrefutable pattern.
fn G(n: i32, 5);
```
When a set of patterns have no ordering or tie-breaker, it is an error for them to overlap unless there is a unique best match for any value that matches more than one pattern. However, this situation does not apply to any current language rule:
- For match statements, patterns are matched top-down, so overlap is permitted.
- We do not yet have an approved design for overloaded functions, but it is anticipated that declaration order will be used in that case too.
- For a set of impls that match a given impl lookup, argument deduction is used rather than pattern matching, but impls with the same type structure are an error unless a match_first declaration is used to order the impls.

Alternatives considered

Pattern usage

This section is a skeletal design, added to support the overview. It should not be treated as accepted by the core team; rather, it is a placeholder until we have more time to examine this detail. Please feel welcome to rewrite and update as appropriate.

Pattern match control flow

The most powerful form and easiest to explain form of pattern matching is a dedicated control flow construct that subsumes the switch of C and C++ into something much more powerful, match. This is not a novel construct, and is widely used in existing languages (Swift and Rust among others) and is currently under active investigation for C++. Carbon's match can be used as follows:

fn Bar() -> (i32, (f32, f32));
fn Foo() -> f32 {
  match (Bar()) {
    case (42, (x: f32, y: f32)) => {
      return x - y;
    }
    case (p: i32, (x: f32, _: f32)) if (p < 13) => {
      return p * x;
    }
    case (p: i32, _: auto) if (p > 3) => {
      return p * Pi;
    }
    default => {
      return Pi;
    }
  }
}

There is a lot going on here. First, let's break down the core structure of a match statement. It accepts a value that will be inspected, here the result of the call to Bar(). It then will find the first case that matches this value, and execute that block. If none match, then it executes the default block.

Each case contains a pattern. The first part is a value pattern ((p: i32, _: auto) for example) optionally followed by an if and boolean predicate. The value pattern has to match, and then the predicate has to evaluate to true for the overall pattern to match. Value patterns can be composed of the following:

An expression (42 for example), whose value must be equal to match.
An identifier to bind the value to, followed by a colon (:) and a type (i32 for example). An underscore (_) may be used instead of the identifier to discard the value once matched.
A tuple destructuring pattern containing a tuple of value patterns ((x: f32, y: f32)) which match against tuples and tuple-like values by recursively matching on their elements.
An unwrapping pattern containing a nested value pattern which matches against a variant or variant-like value by unwrapping it.

In order to match a value, whatever is specified in the pattern must match. Using auto for a type will always match, making _: auto the wildcard pattern.

Guards

We allow cases within a match statement to have guards. These are not part of pattern syntax, but instead are specific to case syntax:

case ::= case pattern [if expression]? => block

A guard indicates that a case only matches if some predicate holds. The bindings in the pattern are in scope in the guard:

match (x) {
  case (m: i32, n: i32) if m + n < 5 => { return m - n; }
}

For consistency, this facility is also available for default clauses, so that default remains equivalent to case _: auto.