data Exp where
    EVar : Char -> Exp
    EAbs : Char -> Exp -> Exp
    EApp : Exp -> Exp -> Exp
    EInt : Int -> Exp
    EAdd : Exp -> Exp -> Exp

data Val where
    VInt     : Int -> Val
    VClosure : List (Pair Char Val) -> Char -> Exp -> Val

lookup : Char -> List (Pair Char Val) -> Val
lookup x cxt = case cxt of
        Cons p ps => case p of
            Pair y v => case (asciiCode x) == (asciiCode y) of
                True  => v
                False => lookup x ps

insert : Char -> Val -> List (Pair Char Val) -> List (Pair Char Val)
insert x v cxt = Cons (Pair x v) cxt

eval : List (Pair Char Val) -> Exp -> Val
eval cxt exp = case exp of
    EVar x => case lookup x cxt of
        VInt i             => VInt i
        VClosure delta x e => eval delta e
    EAbs x e => VClosure cxt x e
    EApp e1 e2 => case eval cxt e1 of
        VClosure delta x f => 
            let v = VClosure cxt x e2 in
            eval (insert x v delta) f
    EInt i => VInt i
    EAdd e1 e2 => 
        let i1 = case eval cxt e1 of { VInt i => i } in 
        let i2 = case eval cxt e2 of { VInt i => i } in 
        VInt (i1 + i2)

exp = EApp (EAbs 'x' (EVar 'x')) (EApp (EAbs 'x' (EAdd (EVar 'x') (EInt 100))) (EInt 200))
-- (λ x . x) (λ x . x + 100) 200

main : Int
main = case eval Nil exp of
           VInt i => i