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.) AntC