Dart function argument can't be assigned even though it is the same type

1
votes

I have this code:

void baz(String s) {
}

void bar<T>(T val, void Function(T) encode) {
}

void foo() {
  (<String>(val) => bar<String>(val, baz))("foo");
}

However it gives this error:

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String)'. dart(argument_type_not_assignable)

A confusing error to say the least! What is going on here?

1
dart

1 Answers

1
votes

Dart type parameters are kind of strange since you can use reserved keywords and types as names for these parameters. In this case, <String>(val) => ... is actually using String as the name of a type argument in the context of the closure, conflicting with the class String in dart:core. If you drop the type argument to your closure in foo, this code should work and the type of val in bar will still be String. If you want to be sure, you can explicitly type the val parameter in the closure:

void foo() {
  ((String val) => bar<String>(val, baz))("foo");
}

Also, the type parameter to bar can be left out as type inference will be able to infer the type T is String:

void foo() {
  ((String val) => bar(val, baz))("foo");
}