Re: [Haskell-beginners] Writing a custom pop function for a stack data type

On Tue, Mar 12, 2013 at 11:06:35AM +0000, doaltan wrote:

Actually I'm getting the error with this :

data Stack = Empty | Elem Char Stack deriving Show

pophead :: Stack -> Char pophead Empty = Empty pophead (Elem x stack) = x

This code will result in a type error, but not the one you said. Try compiling this exact code and see what error you get. -Brent

________________________________ From: divyanshu ranjan To: doaltan ; The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Sent: Tuesday, 12 March 2013, 12:53 Subject: Re: [Haskell-beginners] Writing a custom pop function for a stack data type

You have declared new data type mystack not Stack, so haskell compiler could not find Stack data type and its constructors. Secondly data type in Haskell need to be start with capital letters like

data Mystack = Empty | Elem Char Mystack deriving Show then correct Function definition is  pophead :: Mystack -> Char

Regards Divyanshu 

On Tue, Mar 12, 2013 at 4:12 PM, doaltan wrote:

Hi I have such a stack data structure: 

...

datamystack =Empty |Elem Char mystack derivingShow

I'm trying to get the head of the stack using this: pophead :: Stack -> Char pophead Empty = Empty pophead (Element x stack) = x And I'm getting this error for the last sentence of the function : Not in scope: data constructor `Stack' Can you tell me how to fix it? Thanks.

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