{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}

module Interpreter.Interpreter ( evalTopLevel ) where

import Interpreter.AST
import Control.Monad.Except
import Data.IORef
import Types
import Utils

evalTopLevel :: TopLevel -> KLContext Env KLValue
evalTopLevel (SE se) = reduceSExpr [] se >>= eval []
evalTopLevel (Defun name args body) = evalDefun name args body

evalDefun :: Symbol -> ParamList -> SExpr -> KLContext Env KLValue
evalDefun name args body = do
  body' <- reduceSExpr args body
  let f = topLevelContext name (length args) body'
  insertFunction name f                 
  return (Atom (UnboundSym name))

topLevelContext :: Symbol -> Int -> RSExpr -> ApplContext
topLevelContext name 0 e = PL name (eval [] e)
topLevelContext name n e = Func name (buildContext n [])
    where buildContext 1 vals = Context (\x -> eval ((n-1,x):vals) e)
          buildContext m vals = PartialApp fn
              where fn x = buildContext (m-1) ((n-m,x):vals)

eval :: Bindings -> RSExpr -> KLContext Env KLValue
eval vals (RDeBruijn db)    = lookupVal db vals
eval vals (RFreeze e)       = evalFreeze vals e
eval vals (RTrapError e tr) = evalTrapError vals e tr
eval vals (RLambda i e)     = evalLambda vals i e
eval vals (RIf c t f)       = evalIf vals c t f
eval vals (RApplDir f as)   = evalApplDir vals f as
eval vals (RApplForm a as)  = evalApplForm vals a as
eval vals (RLit a)          = return (checkForBooleans a)
eval vals REmptyList        = return (Atom Nil)

evalTrapError :: Bindings -> RSExpr -> RSExpr -> KLContext Env KLValue
evalTrapError vals e tr = eval vals e `catchError` handleError
    where handleError e =
              eval vals tr >>= \case
                   ApplC (Func name f) -> apply (Func name f) [Excep e]
                   Atom (UnboundSym n) -> do
                     f <- fromIORef =<< functionRef n
                     apply f [Excep e]                     
                   _ -> throwError "exception handler must be a function"
                   
evalFreeze :: Bindings -> RSExpr -> KLContext Env KLValue
evalFreeze vals e = return (ApplC (PL ": thunk" (eval vals e)))

evalLambda :: Monad m => Bindings -> DeBruijn -> RSExpr -> m KLValue
evalLambda vals i = return . ApplC . Func ": lambda" . evalLambda' vals i
    where evalLambda' vals i e
              | RLambda i' e' <- e = PartialApp (contLambda i' e')
              | otherwise = Context termLambda
              where termLambda x = eval (addVal i vals x) e
                    contLambda i' e' x = evalLambda' (addVal i vals x) i' e'

evalApplDir' :: Bindings -> ApplContext -> [RSExpr] -> KLContext Env KLValue
evalApplDir' vals ac args = mapM' (eval vals) args >>= apply ac

evalApplDir :: Bindings -> IORef ApplContext -> [RSExpr] -> KLContext Env KLValue
evalApplDir vals ref args = do
  ac <- fromIORef ref
  mapM' (eval vals) args >>= apply ac

evalApplForm :: Bindings -> RSExpr -> [RSExpr] -> KLContext Env KLValue
evalApplForm vals (RLit (UnboundSym name)) args =
    functionRef name >>= \ac -> evalApplDir vals ac args    
evalApplForm vals form args =
    eval vals form >>= \case
         ApplC ac -> evalApplDir' vals ac args
         Atom (UnboundSym n) -> evalApplForm vals (RLit (UnboundSym n)) args
         v -> exceptionV "(form . args): form evaluates to" v

evalIf :: Bindings -> RSExpr -> RSExpr -> RSExpr -> KLContext Env KLValue
evalIf vals c t f =
    eval vals c >>= \case
         Atom (B True)  -> eval vals t
         Atom (B False) -> eval vals f
         _              -> throwError "(if c t f): c must evaluate to boolean"