On Sat Apr 29 02:23:10 UTC 2017, Richard Eisenberg wrote:

I'm not quite sure what a restriction on (~) might be,

Thanks Richard, I was thinking that FunDeps are restricted to bare type vars. I can't write either of these:

class C a b c | a -> (b, c) -- per my O.P. (~)

class C a b c | a -> (b c)

So it's the one place where type vars `b c` like this:

class C a b c | a -> b c

doesn't mean applying `b` to `c`.

but (~) is effectively declared as

...

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

So I agree with your observations.

Aha! So I could equivalently go:

class EqC a b | a -> b, b -> a

class (EqC a (b, c)) => C a b c where ... -- needs FlexibleInstances

And that does indeed work equivalently to the (~). Again, I haven't put FunDeps on class `C`. So should I reasonably have known that a superclass constraint with FunDeps on the superclass induces FunDeps on the sub-class without explicitly needing to declare so? (I'm not complaining, more surprised/impressed.) AntC

...

On Apr 27, 2017, at 8:14 PM, Anthony Clayden 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.h...

...

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.)

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