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 ________________________________ 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.

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Hi, Dnia 2013-03-12, wto o godzinie 11:06 +0000, doaltan pisze:

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

pophead should return Char. You can't return Stack if you defined the type of pophead as Stack -> Char. You can try to use Maybe here and return Maybe Char (Just x or Nothing) or you can use error function to raise an error. You can use _ instead of stack like this: pophead (Elem x _) = x Emanuel