[Haskell-beginners] Implementing Read

11 Sep 2011

      I'm having some trouble defining an implementation for "Read". I know
I could model a card more simply as an unconstrained tuplet, but I'm
trying to learn new things about haskell as I go.

module Card.Card (
	Suit(..)
	,Card(..)
) where

data Suit = Club | Spade | Diamond | Heart
	deriving (Eq, Ord, Show, Read, Bounded, Enum)

data Card = Card { suit :: Suit
		, rank :: Int
                 }
	deriving (Ord, Bounded, Eq)

instance Show Card where
	show Card { suit = Club, rank = a } = show a ++ "c"
	show Card { suit = Spade, rank = a } = show a ++ "s"
	show Card { suit = Heart, rank = a } = show a ++ "h"
	show Card { suit = Diamond, rank = a } = show a ++ "d"

instance Read Card where
	readsPrec (a:b) = Card{ suit=(toSuit a), rank=(read b) }
		where
		toSuit s
			| s=='c' = Club
			| s=='s' = Spade
			| s=='d' = Diamond
			| s=='h' = Heart

The error is:
Card/Card.hs:24:12:
    Couldn't match expected type `Int' against inferred type `[a]'
    In the pattern: a : b
    In the definition of `readsPrec':
        readsPrec (a : b)
                    = Card {suit = (toSuit a), rank = (read b)}
                    where
                        toSuit s | s == 'c' = Club
                                 | s == 's' = Spade
                                 | s == 'd' = Diamond
                                 | s == 'h' = Heart
    In the instance declaration for `Read Card'

Where am I going wrong?

Thanks,
Peter

[Haskell-beginners] Implementing Read

Peter Hall