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Today's Topics:
1. No instance for (Show a) (Ovidiu Deac)
2. Re: No instance for (Show a) (Daniel Fischer)
----------------------------------------------------------------------
Message: 1
Date: Sun, 31 Jul 2011 12:19:36 +0300
From: Ovidiu Deac <[email protected]>
Subject: [Haskell-beginners] No instance for (Show a)
To: beginners <[email protected]>
Message-ID:
<CAKVsE7uQnESvYgkm=Hw3zeemGRvAJRjAXoSaLKaHJMHwLnn1=w...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
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) ++ ")"
------------------------------
Message: 2
Date: Sun, 31 Jul 2011 11:37:57 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] No instance for (Show a)
To: [email protected]
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="utf-8"
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) ++ ")"
>
------------------------------
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