[Haskell-cafe] Why does Enum succ and pred functions throw exception

[Richard O'Keefe]

On 21/06/2012, at 9:11 PM, Rouan van Dalen wrote:
> I was hoping to have some functions like:
> 
>   safeSucc :: (Enum a) => a -> Maybe a
> 

Types that are instances of Enum don't necessarily have bounds.
It always struck me as odd that Enum doesn't extend Ord.
There's a reason given for not having Bounded extend Ord,
which doesn't really apply to a class having fromEnum :: a -> Int.
Testing whether an enum bound is at a limit is thus a bit tricky.

isMaxBound, isMinBound :: (Enum t, Bounded t) => t -> Bool

isMaxBound x = fromEnum x == fromEnum (maxBound `asTypeOf` x)

isMinBound x = fromEnum x == fromEnum (minBound `asTypeOf` x)

safeSucc, safePred :: (Enum t, Bounded t) => t -> Maybe t

safeSucc x = if isMaxBound x then Nothing else Just (succ x)

safePred x = if isMinBound x then Nothing else Just (pred x)