Edit Question - KnowledgeHub

Title Body

I've been getting into the nitty gritty of the haskell typesystem and trying to get at the fine points of type classes. I've learned a heap, but I'm hitting a wall on the following pieces of code. Using these Class and Instance definitions: class Show a => C a where f :: Int -> a instance C Integer where f x = 1 instance C Char where f x = if x < 10 then 'c' else 'd' Why is it that this passes the type checker: g :: C a => a -> Int -> a g x y = f y yes :: C a => a -> Int -> String yes x y = show (g x y) but this doesn't? g :: C a => a -> Int -> String g x y = show(f y) I find the second alternative much more readable, and it seems to be only a minor difference (note the type signatures). However, trying to get that past the typechecker results in: *Main> :l typetests.hs [1 of 1] Compiling Main ( typetests.hs, interpreted ) typetests.hs:11:14: Ambiguous type variable `a0' in the constraints: (C a0) arising from a use of `f' at typetests.hs:11:14 (Show a0) arising from a use of `show' at typetests.hs:11:9-12 Probable fix: add a type signature that fixes these type variable(s) In the first argument of `show', namely `(f y)' In the expression: show (f y) In an equation for `g': g x y = show (f y) Failed, modules loaded: none. And I don't understand why. Note: Please don't ask "What are you trying to do?" I would hope it is obvious that I'm just messing around in an abstract context in order to probe the way this language works. I have no goal in mind other than learning something new. Thanks

Tags (comma-separated) Cancel