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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