Lambda calculus in less than 20 lines of code.

lambda.hs

type Var = String
type Env = [(Var, Term)]
data Term = Var Var
          | Lambda Var Term
          | Apply Term Term
          | Closure Env Var Term
            deriving(Show)

varLookup ((v,t):env) var = if var == v then
                           t
                         else varLookup env var

extend env v t = ((v,t):env)

eval env (Var v) = varLookup env v
eval env (Lambda v t) = Closure env v t
eval env (Apply t0 t1) = apply (eval env t0) (eval env t1)

apply (Closure e v t0) t1 = eval (extend e v t1) t0