Pattern types

Contents

Cast
Constant
Identifier
List
- Rest element
Logical-and
Logical-or
Map
Null-assert
Null-check
Object
Parenthesized
Record
Relational
Variable
Wildcard

This page is a reference for the different kinds of patterns. For an overview of how patterns work, where you can use them in Dart, and common use cases, visit the main Patterns page.

Cast

foo as String

A cast pattern lets you insert a type cast in the middle of destructuring, before passing the value to another subpattern:

(num, Object) record = (1, "s");
var (i as int, s as String) = record;

Cast patterns will throw if the value doesn’t have the stated type. Like the null-assert pattern, this lets you forcibly assert the expected type of some destructured value.

Constant

123, null, 'string', math.pi, SomeClass.constant, const Thing(1, 2), const (1 + 2)

Constant patterns match when the value is equal to the constant:

switch (number) {
  // Matches if 1 == number.
  case 1: // ... 
}

You can use simple literals and references to named constants directly as constant patterns:

Number literals (123, 45.56)
Boolean literals (true)
String literals ('string')
Named constants (someConstant, math.pi, double.infinity)
Constant constructors (const Point(0, 0))
Constant collection literals (const [], const {1, 2})

More complex constant expressions must be parenthesized and prefixed with const (const (1 + 2)):

// List or map pattern:
case [a, b]: // ...

// List or map literal:
case const [a, b]: // ...

Identifier

foo, _

Identifier patterns may behave like a constant pattern or like a variable pattern, depending on the context where they appear:

Declaration context: declares a new variable with identifier name: var (a, b) = (1, 2);
Assignment context: assigns to existing variable with identifier name: (a, b) = (3, 4);

Matching context: treated as a named constant pattern (unless its name is _):

const c = 1;
switch (2) {
  case c: print('match $c');
  default: print('no match'); // Prints "no match".
 }

Wildcard identifier in any context: matches any value and discards it: case [_, var y, _]: print('The middle element is $y');

List

[subpattern1, subpattern2]

A list pattern matches values that implement List, and then recursively matches its subpatterns against the list’s elements to destructure them by position:

switch (obj) {
  // Matches if obj is a list with two elements.
  case [a, b]: // ... 
}

List patterns require that the number of elements in the pattern match the entire list. You can, however, use a rest element as a place holder to account for any number of elements in a list.

Rest element

List patterns can contain one rest element (...) which allows matching lists of arbitrary lengths.

var [a, b, ..., c, d] = [1, 2, 3, 4, 5, 6, 7];
// Prints "1 2 6 7".
print('$a $b $c $d'); 

A rest element can also have a subpattern that collects elements that don’t match the other subpatterns in the list, into a new list:

var [a, b, ...rest, c, d] = [1, 2, 3, 4, 5, 6, 7];
// Prints "1 2 [3, 4, 5] 6 7".
print('$a $b $rest $c $d'); 

Logical-and

subpattern1 && subpattern2

A pair of patterns separated by && matches only if both subpatterns match. If the left branch does not match, the right branch is not evaluated.

Subpatterns in a logical-and pattern can bind variables, but the variables in each subpattern must not overlap, because they will both be bound if the pattern matches:

switch ((1, 2)) {
  // Error, both subpatterns attempt to bind 'b'.
  case (var a, var b) && (var b, var c): // ... 
}

Logical-or

subpattern1 || subpattern2

A logical-or pattern separates subpatterns by || and matches if any of the branches match. Branches are evaluated left-to-right. Once a branch matches, the rest are not evaluated.

var isPrimary = switch (color) {
  Color.red || Color.yellow || Color.blue => true,
  _ => false
};

Subpatterns in a logical-or pattern can bind variables, but the branches must define the same set of variables, because only one branch will be evaluated when the pattern matches.

Map

{"key": subpattern1, someConst: subpattern2}

Map patterns match values that implement Map, and then recursively match its subpatterns against the map’s keys to destructure them.

Map patterns don’t require the pattern to match the entire map. A map pattern ignores any keys that the map contains that aren’t matched by the pattern.

Null-assert

subpattern!

Null-assert patterns match first if the object is not null, then on the value. They permit non-null values to flow through, but throw if the matched value is null.

To ensure null values are not silently treated as match failures, use a null-assert pattern while matching:

List<String?> row = // ...

switch (row) {
  case ['user', var name!]: // ...
    // 'name' is a non-nullable string here.
}

To eliminate null values from variable declaration patterns, use the null-assert pattern:

(int?, int?) position = // ...

var (x!, y!) = position;

To match when the value is null, use the constant pattern null.

Null-check

subpattern?

Null-check patterns match first if the value is not null, and then match the inner pattern against that same value. They let you bind a variable whose type is the non-nullable base type of the nullable value being matched.

To treat null values as match failures without throwing, use the null-check pattern.

String? maybeString = // ... maybeString is nullable with the base type String.
switch (maybeString) {
  case var s?: ...
    // 's' has type non-nullable String here.
}

To match when the value is null, use the constant pattern null.

Object

SomeClass(x: subpattern1, y: subpattern2)

Object patterns check the matched value against a given named type to destructure data using getters on the object’s properties. They are refuted if the value doesn’t have the same type.

switch (shape) {
  // Matches if shape is of type Rect, and then against the properties of Rect.
  case Rect(width: var w, height: var h): // ...
}

The getter name can be omitted and inferred from the variable pattern or identifier pattern in the field subpattern:

// Binds new variables x and y to the values of Point's x and y properties.
var Point(:x, :y) = Point(1, 2);

Object patterns don’t require the pattern to match the entire object. If an object has extra fields that the pattern doesn’t destructure, it can still match.

Parenthesized

(subpattern)

Like parenthesized expressions, parentheses in a pattern let you control pattern precedence and insert a lower precedence pattern where a higher precedence one is expected.

Record

(subpattern1, subpattern2)

(x: subpattern1, y: subpattern2)

Record patterns match a record object and destructure its fields. If the value isn’t a record with the same shape as the pattern, the match fails. Otherwise, the field subpatterns are matched against the corresponding fields in the record.

Record patterns require that the pattern match the entire record. To destructure a record with named fields using a pattern, include the field names in the pattern:

var (myString: foo, myNumber: bar) = (myString: 'string', myNumber: 1);

The getter name can be omitted and inferred from the variable pattern or identifier pattern in the field subpattern. These pairs of patterns are each equivalent:

// Record pattern with variable subpatterns:
var (untyped: untyped, typed: int typed) = // ...
var (:untyped, :int typed) = // ...

switch (obj) {
  case (untyped: var untyped, typed: int typed): // ...
  case (:var untyped, :int typed): // ...
}

// Record pattern wih null-check and null-assert subpatterns:
switch (obj) {
  case (checked: var checked?, asserted: var asserted!): // ...
  case (:var checked?, :var asserted!): // ...
}

// Record pattern wih cast subpattern:
var (field: field as int) = // ...
var (:field as int) = // ...

Relational

== expression

< expression

Relational patterns compare the matched value to a given constant using any of the equality or relational operators: ==, !=, <, >, <=, and >=.

The pattern matches when calling the appropriate operator on the matched value with the constant as an argument returns true.

Relational patterns are useful for matching on numeric ranges, especially when combined with the logical-and pattern:

String asciiCharType(int char) {
  const space = 32;
  const zero = 48;
  const nine = 57;

  return switch (char) {
    < space => 'control',
    == space => 'space',
    > space && < zero => 'punctuation',
    >= zero && <= nine => 'digit'
    // Etc...
  }
}

Variable

var bar, String str, final int _

Variable patterns bind new variables to values that have been matched or destructured. They usually occur as part of a destructuring pattern in order to capture a destructured value.

The variables are in scope in a region of code that is only reachable when the pattern has matched.

switch ((1, 2)) {
  // 'var a' and 'var b' are variable patterns that bind to 1 and 2, respectively.
  case (var a, var b): // ...
  // 'a' and 'b' are in scope in the case body.
}

A typed variable pattern only matches if the matched value has the declared type, and fails otherwise:

switch ((1, 2)) {
  // Does not match.
  case (int a, String b): // ...
}

You can use a wildcard pattern as a variable pattern.

Wildcard

_

A pattern named _ is a wildcard, either a variable pattern or identifier pattern, that doesn’t bind or assign to any variable.

It’s useful as a placeholder in places where you need a subpattern in order to destructure later positional values:

var list = [1, 2, 3];
var [_, two, _] = list;

A wildcard name with a type annotation is useful when you want to test a value’s type but not bind the value to a name:

switch (record) {
  case (int _, String _):
    print('First field is int and second is String.');
}