Illegal type signature in instance declaration

Question

I'm getting the Illegal type signature in instance declaration error and I have no idea why it is popping up for my program. Indentation seems right, etc. I hope you can help me.

class Game g s | g -> s where
  findPossibleMoves :: Player -> g -> [(s,g)]
  identifyWinner :: g -> Player -> Maybe Player

instance Game HForest HStrategy where
  identifyWinner :: HForest -> Player -> Maybe Player
  identifyWinner ts p = getWinner $ getLeaves ts

  findPossibleMoves :: Player -> HForest -> [(HStrategy, HForest)]
  findPossibleMoves p ts = map (\s -> (s,move s ts)) $ getStrategies p ts

The error is :

Illegal type signature in instance declaration:
  findPossibleMoves :: Player -> HForest -> [(HStrategy, HForest)]
(Use InstanceSigs to allow this)
In the instance declaration for `Game HForest HStrategy'

Reid Barton Reid Barton · Accepted Answer · 2016-01-09T22:39:16

You have a type signature in an instance declaration. That's illegal in standard Haskell. You can enable the InstanceSigs extension (put {-# LANGUAGE InstanceSigs #-} at the top of your file) to allow it. Or just delete the type signature.

Illegal type signature in instance declaration

1 Answers