Compiler cannot unify types of a GADT

[regiskuckaertz]

Compiler version

3.0.0-RC3

Minimized code

enum Func[-A, +B] {
  case Double extends Func[Int, Int]
  case ToString extends Func[Float, String]

  def run: A => B = this match {
    case Double => (x: Int) => x * 2
    case ToString => (x: Float) => x.toString
  }
}

Output

// The compiler complains at each case:
Found: Int
Required: B
// and
Fount: String
Required: B

Expectation

The compiler should be able to unify B with Int or String depending on the case being evaluated.

[LPTK]

Workaround:

enum Func[-A, +B] {
  case Double() extends Func[Int, Int]
  case ToString() extends Func[Float, String]

  def run: A => B = this match {
    case Double() => (x: Int) => x * 2
    case ToString() => (x: Float) => x.toString
  }
}

Currently, GADTs do not do any reasoning about singletons, which is a big limitation. I believe @Linyxus is already working on fixing that.

[regiskuckaertz]

Oh nice! Thank you, that works for me. I'll keep this one opened for tracking.

[regiskuckaertz]

@abgruszecki 🙇