Question

Consider the following method to create a start-at-1 enumeration in Haskell:

data Level = Lower | Middle | Upper
    deriving (Show, Eq, Ord)

instance Enum Level where
    toEnum 1  = Lower
    toEnum 2  = Middle
    toEnum 3  = Upper

    fromEnum Lower  = 1
    fromEnum Middle = 2
    fromEnum Upper  = 3

instance Bounded Level where
    minBound = Lower
    maxBound = Upper

I’d rather not do the following:

data Level = DontUseThis | Lower | Middle | Upper
    deriving (Show, Eq, Ord)

If not, is there a more straightforward way to do this?

Answer 1

First of all, you don’t need to define the Bounded instance yourself. If you add Bounded to the list of derived typeclasses you should get identical behavior.

Secondly, the most straightforward way I can think of to accomplish this is to simply derive Enum and then define your own translation functions. So something like this:

data Level = Lower | Middle | Upper
    deriving (Show, Eq, Ord, Bounded, Enum)

toEnum' x = toEnum (x - 1)
fromEnum' x = (fromEnum x) + 1