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Today's Topics:
1. Equality error (PATRICK BROWNE)
2. Re: Help with improving a program (Florian Gillard)
----------------------------------------------------------------------
Message: 1
Date: Thu, 10 Jul 2014 13:38:00 +0100
From: PATRICK BROWNE <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Equality error
Message-ID:
<cagflrkdta79j2+ej80+cxj8hg6dxodo4bbdv7u8hssqj3ty...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
{-
Hi,
I am trying to get some Haskell code to run from a research paper[1]
The relevant fragment is shown below. The issue is that I cannot get the
code to run using the original Eq instance.
I have replaced the Eq instance with a set of deriving statements on the
data types and this works fine.
But I am anxious to know why I get the error when using the original code.
Also, it seems reasonable to be able to define equality based on names.
With the original Eq Instance I get the following error:
Context reduction stack overflow;...
In the expression: elem car cars
In an equation for `element': element car cars = elem car cars
In the instance declaration for `Collection Cars Car'
I am not too sure how to interpret this message.
Any clarification of the error and possible fix would be appreciated.
Thanks,
Pat
[1] ifgi.uni-muenster.de/~sumitsen/sen_Font05.pdf
-}
{-# LANGUAGE MultiParamTypeClasses,
FlexibleInstances,FunctionalDependencies,
TypeSynonymInstances,UndecidableInstances,TypeSynonymInstances
,OverlappingInstances #-}
import Data.List
class Collection collection single where
empty :: collection
addOne :: single -> collection -> collection
remove :: single -> collection -> collection
element :: single -> collection -> Bool
doToAll :: (single -> single) -> collection -> collection
class Named object name | object -> name where
name :: object -> name
-- **** in original code ****
-- All named objects can be compared for equality if names can
instance (Eq name, Named object name) => Eq object where
object1 == object2 = (name object1) == (name object2)
-- could add (Eq, Show) here
data Edge = Edge Node Node deriving Show -- (Eq, Show)
type Name = [Char]
data Node = Node Name deriving Show -- (Eq, Show)
data Car = Car Node deriving Show -- (Eq, Show)
type Cars = [Car]
instance Named Car Name where
name c = "a"
instance Collection Cars Car where
empty = [] -- ::Cars
addOne car cars = car:cars
remove car cars = delete car cars
element car cars = elem car cars
doToAll f cars = map f cars
-- addOne (Car (Node "a")) [Car (Node "a")]
-- addOne (Car (Node "a")) []::Cars
-- addOne (Car (Node "b")) [(Car (Node "a"))]
-- remove (Car (Node "a")) (addOne (Car (Node "b")) [(Car (Node "a"))])
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Message: 2
Date: Thu, 10 Jul 2014 15:24:44 +0200
From: Florian Gillard <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Help with improving a program
Message-ID:
<caglpzluzga8zxnwqmn27+ni1v7fnpdpx+cfguislusbkop3...@mail.gmail.com>
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I don't know if this is any good but here is my attempt to solve this.
I used comprehensions.
It seems to be working fine with the test files :)
On Thu, Jul 10, 2014 at 11:35 AM, Frerich Raabe <[email protected]> wrote:
> On 2014-07-10 11:24, Francesco Ariis wrote:
>
>> On Wed, Jul 09, 2014 at 11:02:14PM -0300, Marcelo Lacerda wrote:
>>
>>> Hi I'm just starting with haskell and want some help with it.
>>>
>>> I tried to solve the Store Credit[1] problem from google code jam just
>>> to practice, the result was a slow code[2] that I find hard to read.
>>>
>>> Can you guys give me some directions on how to improve it?
>>>
>>> [1] - https://code.google.com/codejam/contest/351101/dashboard#s=p0
>>> [2] - http://pastebin.com/jNGxGP5H
>>>
>>>
>> Don't know about efficiency, but I never liked (!!). Maybe computing all
>> pairs+positions in advance using |zip| would be a little better?
>>
>
> Yeah, that's what I went for as well. I'm attaching my solution for
> comparison to this mail.
>
>
> --
> Frerich Raabe - [email protected]
> www.froglogic.com - Multi-Platform GUI Testing
>
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