import EnvSimple
import Parser
import Repl

type Symbol = String

data Expr =
    ENum Integer
  | EId Symbol
  | EPlus  Expr Expr
  | ETimes Expr Expr
  | EApp Symbol Expr
  | ELet Symbol Expr Expr
  deriving Show

type Environment = Env Integer
type FunDefs     = Env (Symbol, Expr)

eval :: FunDefs -> Environment -> Expr -> Integer
eval fdefs env (ENum n)       = n
eval fdefs env (EId id)       = lookup_variable env id
eval fdefs env (EPlus  e1 e2) = eval fdefs env e1 + eval fdefs env e2
eval fdefs env (ETimes e1 e2) = eval fdefs env e1 * eval fdefs env e2
eval fdefs env (EApp f a) =
  let val    = eval fdefs env a    in
  let (p, b) = lookup_variable fdefs f in
  eval fdefs (bind_variable empty_env p val) b
eval fdefs env (ELet x e b) =
  eval fdefs (bind_variable env x (eval fdefs env e)) b

desugar :: FunDefs -> PExpr -> (FunDefs, Expr)
desugar_expr (PNum n)               = ENum n
desugar_expr (PId id)               = EId id
desugar_expr (PPlus  e1 e2)         = EPlus  (desugar_expr e1) (desugar_expr e2)
desugar_expr (PTimes e1 e2)         = ETimes (desugar_expr e1) (desugar_expr e2)
desugar_expr (PMinus e1 e2)         = EPlus  (desugar_expr e1) (ETimes (ENum (-1)) (desugar_expr e2))
desugar_expr (PApp (PId f) [a])     = EApp f (desugar_expr a)
desugar_expr (PDecl (PLet x _ e) b) = ELet x (desugar_expr e) (desugar_expr b)
desugar_expr _                      = error "unsupported syntactic construct\n"

desugar fdefs (PDecl (PLetFun f [(a,_)] _ b) e) =
  desugar (bind_variable fdefs f (a, desugar_expr b)) e
desugar fdefs e                                 = (fdefs, desugar_expr e)

run str = let (fdefs, expr) = desugar empty_env (parse str) in
          ("", show (eval fdefs empty_env expr))

main :: IO ()
main = repl run Nothing