Lambda-calculus beta-reduction algorithm

gistfile1.hs

import Data.Traversable

data Term a = 
      Var a
    | Lambda a (Term a)
    | Apply (Term a) (Term a)

instance Show a => Show (Term a) where
    show (Var v)       = show v
    show (Lambda n t)  = "/" ++ (show n) ++ "." ++ (show t)
    show (Apply t1 t2) = "(" ++ (show t1) ++ " " ++ (show t2) ++ ")"

--instance Traversable (Term a) where
--    traverse f (Var a) = f (Var a)
--    traverse f (Lambda n t) = f (Lambda n (traverse t))
--    traverse f (Apply t1 t2) = f (Apply (traverse t1) (traverse t2))

reduce :: Eq a => Term a -> Term a
reduce (Var v)       = Var v
reduce (Lambda a t)  = Lambda a (reduce t)
reduce (Apply t1 t2) = apply (reduce t1) (reduce t2)

apply :: Eq a => Term a -> Term a -> Term a
apply (Lambda param t1) t2 = reduce $ rename param t2 t1
apply t1 t2 = Apply t1 t2

fmap' :: (Term a -> Term a) -> Term a -> Term a 
fmap' f (Var v) = f (Var v)
fmap' f (Lambda v t) = Lambda v (fmap' f t)
fmap' f (Apply t1 t2) = Apply (fmap' f t1) (fmap' f t2)
 
rename :: Eq a => a -> Term a -> Term a -> Term a
rename a t1 t2 = fmap' replace t2
    where replace = \(Var x) -> if x == a then t1 else (Var x)

how do you run it?
I mean function call and arguments stuff