import qualified Table
import EnvSimple
import Parser
import Repl

type Symbol = String

data Value =
    VNum Integer
  | VCtor Symbol Int [Value]
  | VRec (Table.T Symbol Value)
  | VFun Symbol Environment Expr
  | VSuspended Environment Expr

data Expr =
    EId Symbol
  | ENum Integer
  | EPlus  Expr Expr
  | ETimes Expr Expr
  | EFun Symbol Expr
  | EApp Expr Expr
  | ELet Symbol Expr Expr
  | ERecord [(Symbol, Expr)]
  | EMember Expr Symbol
  | ETypeVar [(Symbol, Int)] Expr
  | ECase Expr [(Pattern, Expr)]
  deriving Eq

data Pattern = ElsePat | VarPat Symbol | CtorPat Symbol [Symbol] | NumPat Integer
  deriving Eq

type Environment = Env Value

-- evaluate suspended subexpressions
force :: Value -> Value
force (VSuspended env e) = force (eval env e)
force v                  = v

force_nested :: Int -> Value -> Value
force_nested 0 v = v
force_nested n (VSuspended env e) = force_nested (n-1) (eval env e)
force_nested n (VNum k)           = VNum k
force_nested n (VCtor c ar args)  = VCtor c ar (map (force_nested n) args)
force_nested n (VRec fields)      = VRec (Table.map (\_ v -> force_nested n v) fields)
force_nested n (VFun x env e)     = VFun x env e

eval_strict env e = force (eval env e)

eval :: Environment -> Expr -> Value
eval env (ENum n)        = VNum n
eval env (EFun arg body) = VFun arg env body
eval env (EId id)        = lookup_variable env id
eval env (EPlus e1 e2)   =
  case (eval_strict env e1, eval_strict env e2) of
    (VNum x1, VNum x2) -> VNum (x1 + x2)
    _                  -> error "addition of non-numbers"
eval env (ETimes e1 e2)  =
  case (eval_strict env e1, eval_strict env e2) of
    (VNum x1, VNum x2) -> VNum (x1 * x2)
    _                  -> error "multiplication of non-numbers"
eval env (EApp f a)      =
  let susp = VSuspended env a in
  case eval_strict env f of
    VFun p e b         -> eval (bind_variable e p susp) b
    VCtor ctor 0  args -> error "constructor applied to too many arguments"
    VCtor ctor ar args -> VCtor ctor (ar-1) (args ++ [susp])
    _                  -> error "application of non-function"
eval env (ERecord fields) =
  VRec (Table.map (\f e -> eval env e) (Table.of_list fields))
eval env (EMember e f) =
  case eval env e of
    VRec fields -> Table.lookup fields f
    _           -> error "not a record"
eval env (ELet x e b) =
  eval new_env b
  where
    new_env = bind_variable env x v
    v       = eval new_env e
eval env (ETypeVar ctors e) = eval new_env e
  where
    new_env = foldl
                (\env (c,ar) -> bind_variable env c (VCtor c ar []))
                env
                ctors
eval env (ECase e pats) =
    iter pats
    where
      val = eval_strict env e

      iter []                 = error "no matching pattern in case expression"
      iter ((ElsePat,  e):ps) = eval env e
      iter ((VarPat x, e):ps) = case maybe_lookup_variable env x of
          -- check whether x is a constructor or a variable
          Nothing             -> eval (bind_variable env x val) e
          Just (VCtor c 0 []) -> case val of
                                   VCtor d 0 [] | c == d -> eval env e
                                   _                     -> iter ps
      iter ((NumPat n, e):ps) = case val of
          VNum k | n == k -> eval env e
          _               -> iter ps
      iter ((CtorPat c xs,e):ps) = case val of
          VCtor d 0 args | c == d && length xs == length args ->
            eval
              (foldl (\env (x,a) -> bind_variable env x a) env (zip xs args))
              e
          _ -> iter ps

desugar :: PExpr -> Expr
desugar (PId id)              = EId id
desugar (PNum n)              = ENum n
desugar (PPlus  e1 e2)        = EPlus  (desugar e1) (desugar e2)
desugar (PTimes e1 e2)        = ETimes (desugar e1) (desugar e2)
desugar (PMinus e1 e2)        = EPlus  (desugar e1) (ETimes (ENum (-1)) (desugar e2))
desugar (PEqual e1 e2)        = ECase (desugar (PMinus e1 e2))
                                  [ (NumPat 0, EId "%True"),
                                    (ElsePat,  EId "%False") ]
desugar (PFun []           b) = EFun "()" (desugar b)
desugar (PFun [(p,_)]      b) = EFun p (desugar b)
desugar (PFun ((p,_):args) b) = EFun p (desugar (PFun args b))
desugar (PApp f [])           = EApp (desugar f) (EId "()")
desugar (PApp f [a])          = EApp (desugar f) (desugar a)
desugar (PApp f args)         = foldl (\f a -> EApp f a ) (desugar f) (map desugar args)
desugar (PRecord fields)      = ERecord (map (\(f,e) -> (f, desugar e)) fields)
desugar (PMember e f)         = EMember (desugar e) f
desugar (PDecl (PLet x _ e)         b) = ELet x (desugar e) (desugar b)
desugar (PDecl (PLetFun f args _ b) e) = ELet f (desugar (PFun args b)) (desugar e)
desugar (PDecl (PTypeVar _ _ ctors) e) = ETypeVar
                                           (map (\(c,args) -> (c, length args)) ctors)
                                           (desugar e)
desugar (PDecl (PTypeRec _ _ _)     e) = desugar e
desugar (PDecl (PTypeDef _ _ _)     e) = desugar e
desugar (PIf c t e)          = ECase (desugar c)
                                 [ (VarPat "%True", desugar t),
                                   (ElsePat,        desugar e) ]
desugar (PCase e pats)       = ECase (desugar e)
                                     (map (\(p,e) -> (desugar_pat p, desugar e)) pats)
desugar (PListLit es t)      = foldr (\x y -> EApp (EApp (EId "%Cons") (desugar x)) y)
                                 (case t of
                                   Nothing -> EId "%Nil"
                                   Just e  -> desugar e)
                                 es
desugar _                    = error "unsupported syntactic construct\n"

desugar_pat :: PPattern -> Pattern
desugar_pat PElsePat        = ElsePat
desugar_pat (PVarPat x)     = VarPat x
desugar_pat (PCtorPat c xs) = CtorPat c xs
desugar_pat (PNumPat n)     = NumPat n

show_list :: Show a => String -> String -> (a -> String) -> [a] -> String
show_list l r sh [] = ""
show_list l r sh xs = l ++ foldl1 (\ x y -> x ++ ", " ++ y) (map sh xs) ++ r

instance Show Value where
  show (VNum n)         = show n
  show (VCtor c _ args) = c ++ show_list "(" ")" show args
  show (VRec fields)    = show_list "[" "]" (\(f,v) -> f ++ " = " ++ show v)
                            (Table.to_list fields)
  show (VFun a _ b)     = "fun (" ++ a ++ ") { " ++ show b ++ " }"
  show (VSuspended _ b) = "[thunk " ++ show b ++ " ]"

instance Show Expr where
  show (EId x)        = x
  show (ENum n)       = show n
  show (EPlus  e1 e2) = "(" ++ show e1 ++ " + "  ++ show e2 ++ ")"
  show (ETimes e1 e2) = "(" ++ show e1 ++ " * "  ++ show e2 ++ ")"
  show (EFun a b)     = "fun (" ++ a ++ ") { " ++ show b ++ " }"
  show (EApp f a)     = show fn ++ show_list "(" ")" show args where
      (fn, args) = collect_args f [a]
      collect_args (EApp g b) args = collect_args g (b:args)
      collect_args g          args = (g, args)
  show (ERecord fields)    = show_list "[" "]" (\(f,e) -> f ++ " = " ++ show e) fields
  show (EMember e f)       = show e ++ "." ++ f
  show (ELet x b e)        = "let " ++ x ++ " = " ++ show b ++ "; " ++ show e
  show (ECase e pats)      = foldl (++) ("case " ++ show e)
                               (map (\(p,e) -> " | " ++ show p ++ " => " ++ show e) pats)
  show (ETypeVar ctors e)  = foldl (++) ("type ?? =")
                               (map
                                 (\(c,ar) -> " | " ++ c ++
                                               show_list "(" ")" id (replicate ar "_"))
                                 ctors)
                             ++ "; " ++ show e

instance Show Pattern where
  show ElsePat         = "else"
  show (VarPat x)      = x
  show (NumPat n)      = show n
  show (CtorPat c xs)  = c ++ show_list "(" ")" id xs

builtin_ctors =
  foldl (\env (ctor, val) -> bind_variable env ctor val)
    empty_env
    [("()",     VCtor "()"    0 []),
     ("True",   VCtor "True"  0 []),
     ("False",  VCtor "False" 0 []),
     ("Pair",   VCtor "Pair"  2 []),
     ("Nil",    VCtor "Nil"   0 []),
     ("Cons",   VCtor "Cons"  2 []),
     ("%True",  VCtor "True"  0 []),
     ("%False", VCtor "False" 0 []),
     ("%Nil",   VCtor "Nil"   0 []),
     ("%Cons",  VCtor "Cons"  2 [])]

run str = ("", show (force_nested 5 (eval builtin_ctors (desugar (parse str)))))

main :: IO ()
main = repl run Nothing