On Sunday 31 July 2011, 11:19:36, Ovidiu Deac wrote:

For some reason ghc complains about not being able to call show on an Integer (?!?!?)

No, it complains about not being able to call show on a value of arbitrary type. Your instance Show (Stack a) where says you can show stacks of arbitrary type, so nothing can be assumed about the element type, but in the implementation you try to call show on the top element. To make it work, you have to tell GHC that the elements can be shown, instance (Show a) => Show (Stack a) where -- what you have would work. The "(Show a) =>" part says roughly "if elements of the stack can be shown, then the stack can be shown".

Please enlighten me!

ovidiu

This is the hspec: it "shows one element" ( show (push 1 EmptyStack) ≡ "Stack(1)")

...this is the code:

data Stack a = EmptyStack | StackEntry a (Stack a) deriving(Eq)

instance Show (Stack a) where show s = "Stack(" ⊕ (showImpl s) ⊕ ")" where showImpl EmptyStack = "" showImpl (StackEntry x _) = show x

...and this is the error:

src/Stack.hs:12:22: No instance for (Show a) arising from a use of `showImpl' In the first argument of `(++)', namely `(showImpl s)' In the second argument of `(++)', namely `(showImpl s) ++ ")"' In the expression: "Stack(" ++ (showImpl s) ++ ")"