Question

i want to do something like:

succ' :: (Bounded a, Eq a, Enum a) => a -> a
succ' n
| n == (maxBound :: a) = minBound :: a
| otherwise = succ n

but this does not work. how to solve this?

Answer 1

You don’t need the type annotations, and they’re the source of the errors you’re getting:

succ' :: (Bounded a, Eq a, Enum a) => a -> a
succ' n
 | n == maxBound = minBound
 | otherwise = succ n

(This works in Haskell 98 and Haskell 2010, so pretty much any compiler you’ve got lying around .)
Also, I indented the | a little, because they can’t line up with the start of the function; they’re part of the definition of succ', not standalone code.

Here’s some test data:

data Test = A | B | C 
   deriving (Bounded, Eq, Enum, Show)

test = map succ' [A .. C]

I got [B,C,A].