Ah....
So it isn't a closed world....
So how do I stop my instances clashing....?
The "x" in
instance Foo Bar x
is never intended to be Integer.....
Mark Nicholls | Lead broadcast & corporate architect, Programmes & Development - Viacom International Media Networks
A: 17-29 Hawley Crescent London NW1 8TT | e: Nicholls.Mark@vimn.commailto:mik@vimn.com T: +44 (0)203 580 2223
[Description: cid:image001.png@01CD488D.9204D030]
From: Tikhon Jelvis [mailto:tikhon@jelv.is]
Sent: 05 July 2013 2:08 PM
To: Nicholls, Mark
Cc: haskell-cafe
Subject: Re: [Haskell-cafe] newbie question about "Functional dependencies conflict between instance declarations:".....
You're running into the "open world"assumption--anybody could come along and make Integer part of your NotAnInteger class, and there's nothing you can do to stop them. This is a design tradeoff for typeclasses: typeclass instances are always global and are exported to all other modules you use. This means you cannot ensure a type is *not* part of a typeclass. (Or, at the very least, you can't convince GHC of this fact.)
For more information about this, take a look at the following StackOverflow question: http://stackoverflow.com/questions/8728596/explicitly-import-instances
On Jul 5, 2013 8:47 AM, "Nicholls, Mark"
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-}
class Foo x y | x -> y, y -> x instance Foo Integer Integer
That seems to work....and my head seems to say...your created some sort of binary relation between 2 types...and made
data Bar
instance Foo Bar x
Error!....but I'm think I understand this....I can't claim that
class NotAnInteger a
instance (NotAnInteger x) => Foo Bar x
I haven't declared integer to be "NotAnInteger"....so (in a closed world)....this would seem to exclude the contradiction....but... Functional dependencies conflict between instance declarations: instance Foo Integer Integer -- Defined at liam1.lhs:7:12 instance NotAnInteger x => Foo Bar x -- Defined at liam1.lhs:13:12 So i) I clearly don't understand something about the type system. ii) I don't know how to restrict type variables in instance declarations....i.e. how do I use the notion of "Foo" across different combinations of types, without them colliding. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.orgmailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.
