In Dart, given the nullable type `T?`, how do I get the non-nullable one `T`

Question

Given some nullable type T?, how do I get the corresponding non-nullable one T ?

For example:

T? x<T extends int?>(T? value) => value;
Type g<T>(T Function(T) t) => T;
Type type = g(x);
print(type); // Prints "int?"

Now I want to get the non-nullable type. How do I create the function convert so that:

Type nonNullableType = convert(type);
print(nonNullableType); // Prints "int"

Answer 1

In general, you do not. There is no simple way to strip the ? of a type, or destructure types in other ways. (You also can't find the T of type you know is a List<T> at run--time)

If you have the type as a Type object, you can do nothing with it. Using Type object is almost never what you need.

If you have the type as a type parameter, then the type system don't actually know whether it's nullable. Example:

void foo<T>() { ... here T can be nullable or non-nullable ... }

Even if you test null is T to check that the type is actually nullable, the type system doesn't get any smarter, that's not one of the tests that it can derive type information from.

The only types you can improve on are variable types (or rather, the type of a single value currently stored in a variable). So, if you have T x = ...; and you do if (x != null) { ... x is not null here }, you can promote the variable to T&Object, but that's only an intermediate type to allow you to call members on the variable, it's not a real type that you can capture as a type variable or a variable type. It won't help you.

All in all, it can't be done. When you have the nullable type, it's too late, you need to capture it before adding the ?.

What problem are you actually trying to solve?

Answer 2

If you have an instance of T?, I think you could do:

Type nonNullableTypeOf<T>(T? object) => T;

void main() {
  int x = 42;
  int? y;
  print(nonNullableTypeOf(x)); // Prints: int
  print(nonNullableTypeOf(y)); // Prints: int
}

If you have only T? itself (the Type object), then I'm not confident that there's much you can do since what you can do with Type objects is very limited. (And given those limitations, it's not clear that nonNullableTypeOf ultimately would be very useful either.)

A related question: How do I check whether a generic type is nullable in Dart NNBD?

Answer 3

If you have an instance of T?, and you're trying to do something where the expected type is T, you can use use T! wherever dart is showing an error. It is not exactly a conversion from T? to T, its just a shortcut to do a null check.