CODEWITHSUNDEEP
haskellghc
user3657850
user3657850

Reputation: 552

Illegal type signature in instance declaration

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'

Upvotes: 22

Views: 8226

Answers (1)

Reid Barton
Reid Barton

Reputation: 14999

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.

Upvotes: 33

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