Superclass Equality constraints cp FunDeps

[Richard Eisenberg]

I'm not quite sure what a restriction on (~) might be, but (~) is effectively declared as

> class a ~ b | a -> b, b -> a

So I agree with your observations.

Richard


> On Apr 27, 2017, at 8:14 PM, Anthony Clayden <anthony_clayden at clear.net.nz> wrote:
> 
> The docos say [User Guide 10.14.1. on Equality Constraints]
> 
>> Equality constraints can also appear in class and instance
> contexts.
>> The former enable a simple translation of programs using
>> functional dependencies into programs using family
> synonyms instead. 
> http://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/glasgow_exts.html#equality-constraints
> 
> And the forms of constraint seem quite sophisticated.
> I was surprised (pleased) I could do this:
> 
> {-# LANGUAGE   MultiParamTypeClasses, TypeFamilies,
>                             FlexibleInstances #-}
> 
> type family F a
> 
> class (F a ~ (b, c) ) => C a b c   where       -- (b c) !!
>  f1 :: a -> b
>  f2 :: a -> c
> 
> Uses of `f1` happily improve the type for `b`.
> Uses of `f2` happily improve the type for `c`.
> 
> But I didn't declare a Functional Dependency.
> (It seems to do no harm if I add `| a -> b c`.)
> 
> GHC's behaviour seems stronger than a "simple translation".
> It seems entirely equivalent to a FunDep.
> 
> Or is there something I'm missing?
> (I could have overlapping instances,
> but only providing the equations for
> type family `F` are confluent.)
> 
> Are there restrictions on the form of Equality Constraints
> to get them to behave as FunDeps?
> (It's not merely a bare typevar on one side.)
> 
> AntC
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