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darttype-promotiondart-null-safety

Null safety type promotion when assigning non-null value literal

In nullsafety.dartpad.dev if I write the following code:

void main() {
  String? name = 'Bob';
  print(name.length);
}

I get the following compile-time error:

An expression whose value can be 'null' must be null-checked before it can be dereferenced

And the following runtime error:

Property 'length' cannot be accessed on 'String?' because it is potentially null.

The Type promotion on null checks documentation says:

The language is also smarter about what kinds of expressions cause promotion. An explicit == null or != null of course works. But explicit casts using as, or assignments, or the postfix ! operator we’ll get to soon also cause promotion. The general goal is that if the code is dynamically correct and it’s reasonable to figure that out statically, the analysis should be clever enough to do so.

Question

There is no possible way name could be null in the code above. The documentation also says assignments should cause type promotion. Am I misunderstanding type promotion or is this a bug in DartPad?

Clarification

Since a couple of the answers are providing workaround solutions to the error messages, I should clarify that I'm not trying to solve the coding problem above. Rather, I'm saying that I think the code should work as it it. But it doesn't. Why not?

Solution

  • This answer is in response to the bounty that was added to the original question. The bounty reads:

    Please explain how String? is different from String and how type promotion works in Dart.

    String? vs String

    The type String? can contain a string or null. Here are some examples:

    String? string1 = 'Hello world';
String? string2 = 'I ❤️ Dart';
String? string3 = '';
String? string4 = null;

    The type String, on the other hand, can only contains strings (once null safety is a part of Dart, that is). It can't contain null. Here are some examples:

    String string1 = 'Hello world';
String string2 = 'I ❤️ Dart';
String string3 = '';

    If you try to do the following:

    String string4 = null;

    You'll get the compile-time error:

    A value of type 'Null' can't be assigned to a variable of type 'String'.

    The String type can't be null any more than it could be an int like 3 or a bool like true. This is what null safety is all about. If you have a variable whose type is String, you are guaranteed that the variable will never be null.

    How type promotion works

    If the compiler can logically determine that a nullable type (like String?) will never be null, then it converts (or promotes) the type to its non-nullable counterpart (like String).

    Here is an example where this is true:

    void printNameLength(String? name) {
  if (name == null) {
    return;
  }
  print(name.length);
}

    Although the parameter name is nullable, if it actually is null then the function returns early. By the time you get to name.length, the compiler knows for certain that name cannot be null. So the compiler promotes name from String? to String. The expression name.length will never cause a crash.

    A similar example is here:

    String? name;
name = 'Bob';
print(name.length);

    Although name is nullable here, too, the string literal 'Bob' is obviously non-null. This also causes name to be promoted to a non-nullable String.

    The original question was regarding the following:

    String? name = 'Bob';
print(name.length);

    It seems that this should also promote name to a non-nullable String, but it didn't. As @lrn (a Google engineer) pointed out in the comments, though, this is a bug and when null safety comes out, this will also work like the previous example. That is, name will be promoted to a non-nullable String.

    Further reading