Dart nullable records with pattern matching: The matched value of type '(String, int)?' isn't assignable to the required type '(Object?, Object?)'

Question

Given the following function:

(String, int)? getPair() {
  return ('hello', 42);
}

I can access the multiple returns using the fields like so:

final pair = getPair();
if (pair != null) {
  String word = pair.$1;
  int number = pair.$2;
}

However, I'd prefer to destructure the values using some sort of pattern matching. This is what I tried:

final (word, number) = getPair();

But that gives the following error:

The matched value of type '(String, int)?' isn't assignable to the required type '(Object?, Object?)'. Try changing the required type of the pattern, or the matched value type.

How would I change the required type of the pattern or the matched value?

Related reference:

• Dart documentation

Answer 1

You can do that destructuring after the null-check, like you did manually using $1 and $2:

final pair = getPair();
if (pair != null) {
  final (word, number) = pair;
  // ...
}

You can do a not-null assert as part of a declaration pattern:

final ((word, number)!) = getPair();

That will throw if the value is null.

You cannot do a null check as part of a declaration, because declaration patterns must be irrefutable, and a check can fail.

You can do the null check as part of a refutable pattern check, using ?:

if (getPair() case (final word, final number)?) {
  // ...
} // else { "or else, if needed"; }

or

switch (getPair()) {
  case (final word, final number)?: // ....
  // default: "or else, if needed";
}

Since you're then also matching the type in the following destructuring, the ? can be removed.

Or you can combine the two approaches, if you also want the binding of pair:

if (getPair() case final pair?) { // Bind `pair` if it's not `null`.
  final (word, number) = pair;
}