Haskell Typeclass Inspection

Question

I would like to use haskell to implement a game, and would like to use a system of type classes to implement the item system. It would work something like this:

data Wood = Wood Int

instance Item Wood where
  image a = "wood.png"
  displayName a = "Wood"

instance Flammable Wood where
  burn (Wood health) | health' <= 0 = Ash
                     | otherwise    = Wood health'
      where health' = health - 100

where the Item and Flammable classes are something like this:

class Item a where
  image :: a -> String
  displayName :: a -> String

class Flammable a where
  burn :: (Item b) => a -> b

To do this, I would need a way to detect whether a value is an instance of a type class.

The Data.Data module gives a similar functionality so that leads me to believe that this is possible.

Answer 1

Type classes are probably the wrong way to go here. Consider using plain algebraic data types instead, for example:

data Item = Wood Int | Ash

image Wood = "wood.png"
image Ash = ...

displayName Wood = "Wood"
displayName Ash = "Ash"

burn :: Item -> Maybe Item
burn (Wood health) | health' <= 0 = Just Ash
                   | otherwise    = Just (Wood health')
  where health' = health - 100
burn _ = Nothing -- Not flammable

If this makes it too hard to add new items, you can instead encode the operations in the data type itself.

data Item = Item { image :: String, displayName :: String, burn :: Maybe Item }

ash :: Item
ash = Item { image = "...", displayName = "Ash", burn :: Nothing }

wood :: Int -> Item
wood health = Item { image = "wood.png", displayName = "Wood", burn = Just burned }
    where burned | health' <= 0 = ash
                 | otherwise    = wood health'
          health' = health - 100

However, this makes it harder to add new functions. The problem of doing both at the same time is known as the expression problem. There is a nice lecture on Channel 9 by Dr. Ralf Lämmel where he explains this problem more in depth and discusses various non-solutions, well worth the watch if you have time.

There are approaches to solving it, but they are considerably more complex than the two designs I've illustrated, so I recommend using one of those if it fits your needs, and not worrying about the expression problem unless you have to.