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" <nicholls.m...@vimn.com> wrote: > Hello,**** > > ** ** > > I largely don’t know what I’m doing or even trying to do, it is a voyage > into the unknown….but….if I go…**** > > ** ** > > > {-# 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 <Integer,Integer> a member of it… > **** > > ** ** > > Something like that anyway….**** > > ** ** > > Then I go….**** > > ** ** > > > data Bar**** > > ** ** > > > instance Foo Bar x**** > > ** ** > > Error!....but I’m think I understand this….I can’t claim that <Bar,x> is a > member of Foo and <Integer,Integer> is member of Foo and preserve my > functional dependencies, because <Bar,Integer> is now a member of Foo..*** > * > > ** ** > > Bad programmer…….**** > > ** ** > > ** ** > > So how I naively go….**** > > ** ** > > ** ** > > > 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.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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