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

[Nicholls, Mark]

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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