Add combinators

src/ChurchNum.hs

@@ -13,6 +13,7 @@ module ChurchNum (
 ) where
 
 import           Base
+import           Combinators
 
 -- zero :: p2 -> t3 -> t3
 zero = Base.flip constant
@@ -21,8 +22,9 @@ zero = Base.flip constant
 one = apply
 
 -- two :: (t -> t) -> t -> t
--- @help: can't figure out point free version
-two x y = x $ x y
+-- @help: can't figure out point free version (cause s-combinator is not)
+-- two x y = x $ x y
+two = s Base.compose identity
 
 -- inc :: Num a => a -> a
 inc = (+1)

src/Combinators.hs

@@ -0,0 +1,5 @@
+module Combinators where
+
+s f g x = f x (g x) -- S-combinator
+identity x = x -- I-combinator
+constant x y = y -- K -combinator