Chaitin's construction/Combinatory logic

module CL (CL, k, s, apply) where
 
 import Tree (Tree (Leaf, Branch))
 import BaseSymbol (BaseSymbol, kay, ess)
 
 type CL = Tree BaseSymbol 
 
 k, s :: CL
 k = Leaf kay
 s = Leaf ess
 
 apply :: CL -> CL -> CL
 apply = Branch

module CLExt ((>>@)) where
 
 import CL (CL, apply)
 import Control.Monad (Monad, liftM2)
 
 (>>@) :: Monad m => m CL -> m CL -> m CL
 (>>@) = liftM2 apply

module BaseSymbol (BaseSymbol, kay, ess) where
 
 data BaseSymbol = K | S
 
 kay, ess :: BaseSymbol
 kay = K
 ess = S