gistfile1.txt

interface Eq require
  {type T; eq}

interface Eq provide
  type T : Type
  eq : (T, T) -> Bool

module EqInt : Eq where
  type T = Int
  eq = builtIn_eqInt

interface Cmp require
  module C : Eq
  lt : (T, T) -> Bool

interface Cmp default
  type T = C.T
  type Result =
    $lesser $equal $greater
  cmp(x, y) =
    if C.eq(x, y)
    then $equal
    else
      if lt(x, y)
      then $lesser
      else $greater

interface Cmp provide
  {type T; type Result; cmp}

module CmpInt : Cmp where
  module C = EqInt
  lt = builtIn_ltInt

So apparently combining ML's module system with Haskell's typeclasses yields the equivalent of Java's interfaces and classes, minus inheritance. Go to https://github.com/Nenmin/Nail/blob/master/syntax to see what I mean.

I'll have a closer look at Nail.

After getting in contact with my "old" Haskell teacher, I was reminded that O'Haskell had turned into Timber.