Glossary

The following are definitions of terms used across the Dart documentation.

A constant context is a region of code in which it isn't necessary to include the const keyword because it's implied by the fact that everything in that region is required to be a constant. The following locations are constant contexts:

• Everything inside a list, map or set literal that's prefixed by the const keyword. For example:

  dart

  var l = const [/*constant context*/];

• The arguments inside an invocation of a constant constructor. For example:

  dart

  var p = const Point(/*constant context*/);

• The initializer for a variable that's prefixed by the const keyword. For example:

  dart

  const v = /*constant context*/;

• Annotations.

• The expression in a case clause. For example:

  dart

  void f(int e) {
    switch (e) {
      case /*constant context*/:
        break;
    }
  }

Related docs and resources:

• Final and const variables
• Don't use const redundantly

Definite assignment analysis is the process of determining, for each local variable at each point in the code, which of the following is true:

• The variable has definitely been assigned a value (definitely assigned).
• The variable has definitely not been assigned a value (definitely unassigned).
• The variable might or might not have been assigned a value, depending on the execution path taken to arrive at that point.

Definite assignment analysis helps find problems in code, such as places where a variable that might not have been assigned a value is being referenced, or places where a variable that can only be assigned a value one time is being assigned after it might already have been assigned a value.

For example, in the following code the variable s is definitely unassigned when it's passed as an argument to print:

void f() {
  String s;
  print(s);
}

But in the following code, the variable s is definitely assigned:

void f(String name) {
  String s = 'Hello $name!';
  print(s);
}

Definite assignment analysis can even tell whether a variable is definitely assigned (or unassigned) when there are multiple possible execution paths. In the following code the print function is called if execution goes through either the true or the false branch of the if statement, but because s is assigned no matter which branch is taken, it's definitely assigned before it's passed to print:

void f(String name, bool casual) {
  String s;
  if (casual) {
    s = 'Hi $name!';
  } else {
    s = 'Hello $name!';
  }
  print(s);
}

In flow analysis, the end of the if statement is referred to as a join—a place where two or more execution paths merge back together. Where there's a join, the analysis says that a variable is definitely assigned if it's definitely assigned along all of the paths that are merging, and definitely unassigned if it's definitely unassigned along all of the paths.

Sometimes a variable is assigned a value on one path but not on another, in which case the variable might or might not have been assigned a value. In the following example, the true branch of the if statement might or might not be executed, so the variable might or might be assigned a value:

void f(String name, bool casual) {
  String s;
  if (casual) {
    s = 'Hi $name!';
  }
  print(s);
}

The same is true if there is a false branch that doesn't assign a value to s.

The analysis of loops is a little more complicated, but it follows the same basic reasoning. For example, the condition in a while loop is always executed, but the body might or might not be. So just like an if statement, there's a join at the end of the while statement between the path in which the condition is true and the path in which the condition is false.

Related docs and resources:

• Definite assignment specification
• Understanding definite assignment analysis

An umbrella term to refer to top-level functions, local functions, static methods, and instance methods.

Related docs and resources:

• Functions overview
• Instance methods

Irrefutable patterns are patterns that always match. Irrefutable patterns are the only patterns that can appear in irrefutable contexts: the declaration and assignment pattern contexts.

Related docs and resources:

• Places patterns can appear
• Dive into Dart patterns and records

A mixin application is the class created when a mixin is applied to a class. For example, consider the following declarations:

class A {}

mixin M {}

class B extends A with M {}

The class B is a subclass of the mixin application of M to A, sometimes nomenclated as A+M. The class A+M is a subclass of A and has members that are copied from M.

You can give an actual name to a mixin application by defining it as:

class A {}

mixin M {}

class A_M = A with M;

Given this declaration of A_M, the following declaration of B is equivalent to the declaration of B in the original example:

class B extends A_M {}

Related docs and resources:

• Mixins in Dart

Override inference is the process by which any missing types in a method declaration are inferred based on the corresponding types from the method or methods that it overrides.

If a candidate method (the method that's missing type information) overrides a single inherited method, then the corresponding types from the overridden method are inferred. For example, consider the following code:

class A {
  int m(String s) => 0;
}

class B extends A {
  @override
  m(s) => 1;
}

The declaration of m in B is a candidate because it's missing both the return type and the parameter type. Because it overrides a single method (the method m in A), the types from the overridden method will be used to infer the missing types and it will be as if the method in B had been declared as int m(String s) => 1;.

If a candidate method overrides multiple methods, and the function type one of those overridden methods, M_s, is a supertype of the function types of all of the other overridden methods, then M_s is used to infer the missing types. For example, consider the following code:

class A {
  int m(num n) => 0;
}

class B {
  num m(int i) => 0;
}

class C implements A, B {
  @override
  m(n) => 1;
}

The declaration of m in C is a candidate for override inference because it's missing both the return type and the parameter type. It overrides both m in A and m in B, so the compiler needs to choose one of them from which the missing types can be inferred. But because the function type of m in A (int Function(num)) is a supertype of the function type of m in B (num Function(int)), the function in A is used to infer the missing types. The result is the same as declaring the method in C as int m(num n) => 1;.

It is an error if none of the overridden methods have a function type that is a supertype of all the other overridden methods.

Related docs and resources:

• Inheritance in Dart

A part file is a Dart source file that contains a part of directive and is included in a library using the part directive.

Related docs and resources:

• Guidance on libraries and parts
• Use library URIs in part of directives

A type is potentially non-nullable if it's either explicitly non-nullable or if it's a type parameter.

A type is explicitly non-nullable if it is a type name that isn't followed by a question mark (?). Note that there are a few types that are always nullable, such as Null and dynamic, and that FutureOr is only non-nullable if it isn't followed by a question mark and the type argument is non-nullable (such as FutureOr<String>).

Type parameters are potentially non-nullable because the actual runtime type (the type specified as a type argument) might be non-nullable. For example, given a declaration of class C<T> {}, the type C could be used with a non-nullable type argument as in C<int>.

Related docs and resources:

• Nullability and generics

A public library is a library that is located inside the package's lib directory but not inside the lib/src directory.

Related docs and resources:

• Organizing a package's libraries
• Public package directories

A refutable pattern is a pattern that can be tested against a value to determine if the pattern matches the value. If not, the pattern refutes, or denies, the match. Refutable patterns appear in matching contexts.

Related docs and resources:

• Places patterns can appear
• Dive into Dart patterns and records

A subclass is a class that inherits the implementation of another class by using the extends keyword, or by mixin application.

// A is a subclass of B; B is the superclass of A.
class A extends B {}

// B1 has the superclass `A with M`, which has the superclass A.
class B1 extends A with M {}

A subclass relation also implies an associated subtype relation. For example, class A implicitly defines an associated type A which instances of the class A inhabit. So, class A extends B declares not just that the class A is a subclass of B, but also establishes that the type A is a subtype of the type B.

Subclass relations are a subset of subtype relations. When the documentation says "S must be a subtype of T", it's fine for S to be a subclass of T. However, the converse is not true: not all subtypes are subclasses.

Related docs and resources:

• Inheritance in Dart
• Subtype relationship

A subtype relation is where a value of a certain type is substitutable where the value of another type, the supertype, is expected. For example, if S is a subtype of T, then you can substitute a value of type S where a value of type T is expected.

A subtype supports all of the operations of its supertype (and possibly some extra operations). In practice, this means you can assign the value of a subtype to any location expecting the supertype, and all of the methods of the supertype are available on the subtype.

This is true at least statically. A specific API might not allow the substitution at run time, depending on its operations.

Some subtype relations are based on the structure of the type, like with nullable types (for example, int is a subtype of int?) and function types (for example, String Function() is a subtype of void Function()).

Subtypes can also be introduced for classes by implementation or inheritance (direct or indirect):

// A is a subtype of B, but NOT a subclass of B.
class A implements B {}

// C is a subtype AND a subclass of D.
class C extends D {}

Related docs and resources:

• Subclass
• Substituting types

A type parameter of a class (or other type declaration, like a mixin) is said to be covariant when the type as a whole "co-varies" with the actual type argument. In other words, if the type argument is replaced by a subtype then the type as a whole is also a subtype.

For example, the type parameter of the class List is covariant because list types co-vary with their type argument: List<int> is a subtype of List<Object> because int is a subtype of Object.

In Dart, all type parameters of all class, mixin, mixin class, and enum declarations are covariant.

However, function types are different: A function type is covariant in its return type, but the opposite (known as contravariant) in its parameter types. For example, the type int Function(int) is a subtype of the type Object Function(int), but it is a supertype of int Function(Object).

This makes sense if you consider their substitutability. If you call a function with a static type of int Function(int), that function can actually be of type int Function(Object) at runtime. Based on the static type, you expect to be able to pass an int to it. That will be fine since the function actually accepts any Object, and this includes every object of type int. Similarly, the returned result will be of type int, which is also what you expect based on the static type.

Hence, int Function(Object) is a subtype of int Function(int).

Note that everything is turned upside-down for parameter types. In particular, this subtype relation among function types requires that the opposite subtype relation exists for the parameter type. For example, void Function(Object) is a subtype of void Function(int) because int is a subtype of Object.

With a more complex type like List<void Function(int)>, you have to consider the positions in the type. To accomplish this, turn one of the parts of the type into a placeholder, and then consider what happens to the type when different types are placed in that position.

For example, consider List<void Function(_)> as a template for a type where you can put different types in place of the placeholder _. This type is contravariant in the position where that placeholder occurs.

The following illustrates this by substituting Object and int for _. List<void Function(Object)> is a subtype of List<void Function(int)> because void Function(Object) is a subtype of void Function(int) because void is a subtype of void (the return types) and int is a subtype of Object (the parameter types, in the opposite order). Hence, the type at _ varies in the opposite direction of the type List<void Function(_)> as a whole, and this 'opposite direction' by definition makes it a contravariant position.

A covariant position is defined similarly. For example, _ is at a covariant position in the type List<_>, and _ is also at a covariant position in the type _ Function(int).

There is yet another kind of position known as invariant, but it occurs much more rarely so the details are omitted here.

In practice, it's often sufficient to know that the type arguments of a class, mixin, etc. are in a covariant position, and so is the return type of a function type, but the parameter types are in a contravariant position.

Related docs and resources:

• Subtype relationship
• Covariance and contravariance
• The covariant keyword

Unless stated otherwise, the documentation on this site reflects Dart 3.5.3. Page last updated on 2024-10-22. View source or report an issue.