type checking for conditional expression

user_115198 0
Viewed 48

In the following code:

main() {
   print('Starting....');
   int x = 10;
   int y = x > 3 ? 'aaa' : 7;
   print(y);
}

This is the output:

Starting....
Unhandled exception:
type 'String' is not a subtype of type 'int'
#0      main (file:///tmp/main.dart:4:12)
#1      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:301:19)
#2      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

why isn't there a compile time error on the assignment to y? Why only runtime error?

version: Dart VM version: 2.8.4 (stable) (Unknown timestamp) on "linux_x64"

Thanks!

dart
2 Answers

The reason is that Dart does still try to make implicit type casts as default behavior. So in this case, Dart will automatically assume you can cast the String into a int. This behavior can be disabled and will also be disabled by default when non-nullable types are supported:

https://dart.dev/guides/language/analysis-options#enabling-additional-type-checks

You can add the following to analysis_options.yaml (I do also recommend to add implicit-dynamic: false but this is not necessary for this specific issue):

analyzer:
  strong-mode:
    implicit-casts: false

And you will then get an error from the analyzer:

Analyzing bin/stackoverflow.dart...
  error • A value of type 'Object' can't be assigned to a variable of type 'int'. • bin/stackoverflow.dart:4:11 • invalid_assignment
1 error found.

The reason why the error are saying Object is because Dart has analyzed x > 3 ? 'aaa' : 7 into the return type of Object since that is the only thing in common for int and String.

user_19535150

You are trying to parse the string 'aaa' in a type called int. 'aaa' is not a number so error.

String y = x > 3 ? 'aaa' : '7';

or do:

int y = x > 3 ? 2 : 7;
user_68569460
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