Function class

A function value.

The Function class is a supertype of all function types, and contains no values itself. All objects that implement Function have a function type as their runtime type.

The Function type does not carry information about the parameter signatures or return type of a function. To express a more precise function type, use the function type syntax, which is the Function keyword followed by a parameter list, or a type argument list and a parameter list, and which can also have an optional return type.

The function type syntax mirrors the definition of a function, with the function name replaced by the word "Function".

Example:

String numberToString(int n) => "$n";
String Function(int n) fun = numberToString; // Type annotation
assert(fun is String Function(int)); // Type check.
List<String Function(int)> functions = [fun]; // Type argument.

The type String Function(int) is the type of a function that takes one positional int argument and returns a String.

Example with generic function type:

T id<T>(T value) => value;
X Function<X>(X) anotherId = id; // Parameter name may be omitted.
int Function(int) intId = id<int>;

A function type can be used anywhere a type is allowed, and is often used for functions taking other functions, "callbacks", as arguments.

void doSomething(String Function(int) callback) {
  print(callback(1));
}

A function type has all the members declared by Object, since function types are subtypes of Object.

A function type also has a call method with a signature that has the same function type as the function type itself. Calling the call method behaves just as calling the function. This is mainly used to conditionally call a nullable function value.

String Function(int) fun = (n) => "$n";
String Function(int) fun2 = fun.call; // Valid.
print(fun2.call(1)); // Prints "1".

String Function(int)? maybeFun = Random().nextBool() ? fun : null;
print(maybeFun?.call(1)); // Prints "1" or "null".

The Function type has a number of special features which are not visible in this class declaration.

The Function type itself allows any function to be assigned to it, since it is a supertype of any function type, but does not say how the function can be called.

However, a value with the static type Function can still be called like a function.

Function f = (int x) => "$x";
print(f(1)); // Prints "1".

f("not", "one", "int"); // Throws! No static warning.

Such an invocation is a dynamic invocation, precisely as if the function value had been statically typed as dynamic, and is precisely as unsafe as any other dynamic invocation. Checks will be performed at run-time to ensure that the argument list matches the function's parameters, and if not the call will fail with an Error. There is no static type checking for such a call, any argument list is accepted and checked at runtime.

Like every function type has a call method with its own function type, the Function type has a special call member which acts as if it is a method with a function type of Function (which is not a method signature which can be expressed in normal Dart code).

Function fun = (int x) => "$x";

var fun2 = f.call; // Inferred type of `fun2` is `Function`.

print(fun2.call(1)); // Prints "1";

Function? maybeFun = Random().nextBool() ? fun : null;
print(maybeFun?.call(1)); // Prints "1" or "null".