Is there some way to define an Enum in haskell that wraps around?

Question

Consider I was designing a Monopoly game:

data Board = GO | A1 | CC1 | A2 | T1 | R1 | B1 | CH1 | B2 | B3 | 
  JAIL | C1 | U1 | C2 | C3 | R2 | D1 | CC2 | D2 |         


        

Answer 1

There is a disgusting way to define an efficient wrapping Enum instance without doing much by hand.

{-# LANGUAGE MagicHash #-}

import GHC.Exts (Int (..), tagToEnum#, dataToTag# )

-- dataToTag# :: a -> Int#
-- tagToEnum# :: Int# -> a

Now you can write

data Board = ... deriving (Eq, Ord, Bounded)

instance Enum Board where
  fromEnum a = I# (dataToTag# a)

  toEnum x | x < 0 || x > fromEnum (maxBound :: Board) =
    error "Out of range"
  toEnum (I# t) = tagToEnum# t
  succ x | x == maxBound = minBound
         | otherwise == toEnum (fromEnum x + 1)
  pred x ....