[Haskell-beginners] No instance for (Show a)

31 Jul 2011

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

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) ++ ")"

[Haskell-beginners] No instance for (Show a)

Ovidiu Deac