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

[Brent Yorgey]

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 <idivyanshu.ranjan at gmail.com>
> To: doaltan <doaltan at yahoo.co.uk>; The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners at haskell.org> 
> 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 <doaltan at yahoo.co.uk> 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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> >http://www.haskell.org/mailman/listinfo/beginners
> >
> >

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