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Today's Topics:
1. Stack implementation (Uneeb Adeel Agha)
----------------------------------------------------------------------
Message: 1
Date: Sat, 10 Sep 2016 21:07:48 +0000
From: Uneeb Adeel Agha <uneeb.a...@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Stack implementation
Message-ID: <cb003da7-ffde-9f39-7237-49eb0f9dc...@mixmax.com>
Content-Type: text/plain; charset="utf-8"
Hi,
I'm very, very new to Haskell. I am trying to understand the following error,
but I'm having a hard time wrapping my head around the whole type system.
Stack
module Stack (Stack(..)) where import Prelude hiding
(head, tail) class Stack s where empty :: s a isEmpty :: s a
-> Bool cons :: s a -> a -> s a head :: s a -> a tail :: s
a -> s a data ListStack a = LS [a] deriving (Show) instance Stack
ListStack where empty = LS [] isEmpty (LS s) = null s cons
(LS s) x = LS(x:s) head(LS(x:_)) = x tail(LS(x:xs)) = LS xs
Not using Mixmax yet?
The problem is that when I load this code in the interpreter and write>emptyI
get the following error
<interactive>:108:1: No instance for (Show (s0 a0)) arising from a use of
‘print’ The type variables ‘s0’, ‘a0’ are ambiguous
I mean when I write "LS []" it works just fine. But something is wrong with
empty.
Thanks,
Uneeb
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