[Haskell-cafe] Circular enums

Ben Butler-Cole ben at bridesmere.com
Sat Feb 2 18:47:39 EST 2008

[Resend with formatting maybe fixed.]


I'm trying to define functions that allow you to traverse a bounded enumeration, "wrapping" at the start and the end.

My implementation looks like this:

    next, prev :: (Enum a, Bounded a) => a -> a
    next = turn 1
    prev = turn (-1)
    turn :: (Enum a, Bounded a) => Int -> a -> a
    turn n e = toEnum (add (fromEnum (maxBound::a) + 1) (fromEnum e) n)
          add mod x y = (x + y + mod) `rem` mod

Which fails to type check under GHC with this error:

    No instance for (Bounded a)
      arising from use of `maxBound' at Hbot.hs:6:34-41
    Probable fix: add (Bounded a) to the expected type of an expression
    In the expression: maxBound :: a
    In the first argument of `fromEnum', namely `(maxBound :: a)'
    In the first argument of `(+)', namely `fromEnum (maxBound :: a)'

My (clearly flawed) understanding of the signature I've specified for 'turn' means *exactly* that a is Bounded.

Can anyone enlighten me as to where my understanding is going awry and how (whether) I can achieve what I'm trying to do.

Thank you

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