I've got this import Data.Maybe data MyList a = Empty | Cons a (MyList a) deriving (Eq,Ord,Show) data BaconOrIndex = Bacon | Indx Int deriving (Eq,Ord,Show) whereIsBM Empty = Nothing whereIsBM (Cons idx lx) = if (idx == Bacon) then Just 1 else (whereIsBM lx) which I would like to tell me where the Bacon is (index), not just if there's Bacon, which is what it does now. That is, I need this to happen
whereIsBM (Cons (Indx 5) (Cons Bacon (Cons (Indx 2) (Cons (Indx 8) Empty)))) Just 2
So I need to traverse a BaconOrIndex list and count how deep I went to find the Bacon variable. I get the above code to evaluate error-free, but obviously I'm only returning a Just 1 when it sees Bacon. What I need is to have the last part be . . . else (1 + whereIsBM lx) work; but it keeps giving the error Non type-variable argument in the constraint: Num (Maybe a) (Use FlexibleContexts to permit this) • When checking the inferred type whereIsBM :: forall a. (Num a, Num (Maybe a)) => MyList BaconOrIndex -> Maybe a I haven't a clue what this means. Eventually, I'll wrap this in something that handles the Nothing and does fromJust on the alternative. This whole effort is because if I didn't use the Maybe strategy, and said whereIsBM Empty = 0 ... it would never give back 0 if it didn't find Bacon, rather, it would simply return the whole countdown to Empty. What can I do to make Maybe work here? LB