[Haskell-cafe] newbie question about "Functional dependencies conflict between instance declarations:".....

Fri Jul 5 15:22:53 CEST 2013

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 at vimn.com<mailto:mik at vimn.com> T: +44 (0)203 580 2223

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From: Tikhon Jelvis [mailto:tikhon at 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" <nicholls.mark at vimn.com<mailto:nicholls.mark at 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.

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