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To help me learn Applicative Functors and Functors I thought it would be good fun to see how Either is implemented with the typeclasses Functor and Applicative . Obviously I could just go ahead and read the code but I find it more useful to try and implement things myself to get a better understanding of things. FYI I'm trying to implement the Haskell version of the results of this presentation http://applicative-errors-scala.googlecode.com/svn/artifacts/0.6/chunk-html/index.html Anyway, this is what I have so far data Validation a b = Success a | Failure b deriving (Show, Eq) instance Functor (Validation a) where fmap f (Failure x) = Failure x fmap f (Success x) = Success (f x) But whenever I try to run this with ghci I just get the following error message: - [1 of 1] Compiling Main ( t.hs, interpreted ) t.hs:5:35: Couldn't match type `b' with `a1' `b' is a rigid type variable bound by the type signature for fmap :: (a1 -> b) -> Validation a a1 -> Validation a b at t.hs:4:5 `a1' is a rigid type variable bound by the type signature for fmap :: (a1 -> b) -> Validation a a1 -> Validation a b at t.hs:4:5 Expected type: a Actual type: b In the return type of a call of `f' In the first argument of `Success', namely `(f x)' In the expression: Success (f x) t.hs:5:37: Couldn't match type `a' with `a1' `a' is a rigid type variable bound by the instance declaration at t.hs:3:30 `a1' is a rigid type variable bound by the type signature for fmap :: (a1 -> b) -> Validation a a1 -> Validation a b at t.hs:4:5 In the first argume

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