{----

Monadic parser for a simple lambda calculus.

----}

module Lambda where

import Control.Monad.State
import MonParser
import Lexing

--
-- lambda calculus parser
--
data Expr = App Expr Expr
         | Lam String Expr
         | Let String Expr Expr
         | Var String
  deriving Show

expr = atom `chainl1` (return App)

atom = lam +++ local +++ paren +++ var
--atom = lam +++ local +++ var +++ paren

lam = do
    symbol "\\"
    x <- variable
    symbol "->"
    e <- expr
    return $ Lam x e

local = do
    symbol "let"
    x <- variable
    symbol "="
    e <- expr
    symbol "in"
    e' <- expr
    return $ Let x e e'

var = do 
    x <- variable
    return $ Var x


paren = bracket (symbol "(") expr (symbol ")")

variable = identifier ["let", "in"]