dart wrongly assumes value can be nullable

Issue

I have the code

if (WORDLIST[language]==null) throw new Exception("Invalid Language");
List<String> wordlist = WORDLIST[language];

the compiler says

Error: A value of type 'List?' can't be assigned to a variable of type 'List' because 'List?' is nullable and 'List' isn't.

However its both not possible in my code for language to be set to a value that is not valid and just to be safe there is the exception thrown if it gets called somewhere without checking the language exists. How can I get the compiler to recognize this is valid?

Full Code is here(though the exception was added afterwards to try and handle new compiler): https://github.com/mctrivia/bip39-multi/blob/mctrivia-patch-1/lib/src/bip39_multi_base.dart#L80

Solution

Map.operator [] returns a nullable type, period. The compiler doesn't know what WORDLIST[language] will return at runtime; it doesn't know that WORDLIST isn't some Map implementation that returns different values each time operator [] is called. This is the same reason why only local variables can be type promoted.

The typical ways to fix this are:

• Use ! to force the result to be non-nullable:

  if (WORDLIST[language]==null) throw new Exception("Invalid Language");
  List<String> wordlist = WORDLIST[language]!;
  

• Use a local variable:

  var result = WORDLIST[language];
  if (result == null) throw new Exception("Invalid Language");
  List<String> wordlist = result;
  

  Since wordlist is probably already a local variable, you also could just reorder your code:

  List<String>? wordlist = result;
  if (wordlist == null) throw new Exception("Invalid Language");
  // wordlist is now type-promoted to be List<String>.
  

Answered By - jamesdlin