Null-safety: null check not working with type Function

[esotericpig]

I couldn't find an issue about this, but maybe it has already been mentioned.

The below code doesn't compile:

class Tester {
  void Function()? myFunc;
  
  Tester(this.myFunc);
  
  void test() {
    if(myFunc != null) {
      // ERROR: The function can't be unconditionally invoked because it can be 'null'.
      myFunc();
    }
  }
}

The workaround of course:

myFunc!();

It seems like the null-check if-statement should resolve it, just like when using a variable of any other type, but it doesn't work with type Function, but works with other types:

class Tester2 {
  String? name;
  
  void test() {
    if(name != null) {
      // Compiles fine.
      print(name);
    }
  }
}

Thanks

[Levi-Lesches]

This isn't because it's a function, it's because myFunc is a field. Local variables can be promoted, but fields cannot.

Your name example works because you can pass null to print. name isn't being promoted, but print doesn't care. If you tried to access a property of name, or pass it to a function that expects a non-nullable, you'd get an error.

Some examples:

/// Fields do not promote...
class Temp {
  Function? func;
  String? string;
  
  void test() {
    if (func != null) func();  // error
    if (string != null) print(string.length);  // error
    print(string);  // always fine
  }
}

/// ...but local variables do
void main() {
  Function? func;
  String? string;
  if (func != null) func();  // OK
  if (string != null) print(string.length);  // OK
  print(string);  // always fine
}

[esotericpig]

Oh okay, I see. Thanks.

That's quite confusing. I thought fields/properties were okay with null-check. I didn't know that it's only for args/locals. So then it works with args:

  void test(void Function()? myFunc) {
    if(myFunc != null) {
      myFunc();
    }
  }

Well, hopefully it gets added to instance vars as well one day.

[Levi-Lesches]

The reason fields don't promote is because fields are actually getters under the hood, and getters are just functions that can return any value. Specifically, they can return a different value every time they're called. Even final variables aren't safe, because final simply means "this has a getter, but no setter". Subclasses can override the getter with their own logic. The following is hopefully never used in production, but technically could be:

import "dart:math";

class User {
  /// This is null half the time, and non-null the other half.
  String? get username => Random().nextBool() ? "JohnSmith42" : null;
  
  void greet() {
    if (username != null)  // Just because it's non-null here...
      print(username.length);  // ...doesn't mean it's non-null here!
  }
}

Obviously, this is convoluted, but with actual logic like "is the user logged in?", this could happen. And even if you're classes don't do this, someone can extend it and do it themselves.

If you really want promotion without the !, you'd make a local variable that reflects the value of the field, and promote that:

class Temp {
  Function? func;
  String? string;
  
  void test() {
    if (func != null) func();  // error
    final String? string = this.string;
    if (string != null) print(string.length);  // OK
    print(string);  // always fine
  }
}

---

There are a couple of alternatives being discussed:

• Stable getters #1518: A stronger final that means the field will never change (or at least without explicitly setting it)

class User {
  final String? username;  // once declared, never changes
  const User(this.username);

  /// null if [username] is null, but won't change
  final String? get email => username == null ? null : "$username@gmail.com";
   
  void test() {
    if (username != null) print(username.length);  // OK, because it hasn't changed
  }
}

• Shadowing a non-local variable #1514: A local variable that redirects to a field, so it can be promoted:

void test() {
  shadow string;  // actually refers to this.string
  if (string != null) print(string.length);  // OK
  print(string);  // always fine
}

• If-variables #1201: A quick way to make a local variable that holds the value of a field. Today, you would do:

void test() {
  if (var string != null) print(string.length);  // quickly defines a local variable
  print(string);  // always fine
}