[Haskell-cafe] Partial type family application

[Oleg]

If the goal is to write higher-order type families, we can do that
already -- and were able to do for a long time.

For example, please see
        http://okmij.org/ftp/Haskell/TTypeable/TTypeable.hs
that was written in 2012 for
        http://okmij.org/ftp/Haskell/typeEQ.html

It shows the type-level function that checks for a membership in a
type-level using the equality function supplied by a user as the first
argument. Incidentally, here is the definition


type instance Member f x NIL = HFalse
type instance Member f x (h :/ t) = 
  ORELSE (Apply f (x,h)) (CLOS AC_Member (f,x,t))

It was written before the modern syntax for type-level lists and
booleans.

This is an example of the general technique of defunctionalization.