{-# LANGUAGE KindSignatures, GADTs, InstanceSigs, FlexibleContexts, ScopedTypeVariables #-}

module HERMIT.Shell.Interpreter
        ( -- * The HERMIT Interpreter
          Interp
        , interp
        , interpExprH
        ) where

import Control.Monad.State

import Data.Char
import Data.Dynamic
import qualified Data.Map as M

import HERMIT.External
import HERMIT.Parser
import HERMIT.Kure (deprecatedIntToPathT,pathToSnocPath)

import HERMIT.Shell.Types

-- | Interpret an 'ExprH' by looking up the appropriate 'Dynamic'(s) in the provided 'Dictionary', then interpreting the 'Dynamic'(s) with the provided 'Interp's, returning the first interpretation to succeed (or an error string if none succeed).
interpExprH :: MonadState CommandLineState m => [Interp a] -> ExprH -> m a
interpExprH interps e = do 
    dyns <- interpExpr e
    runInterp e dyns interps

runInterp :: Monad m => ExprH -> [Dynamic] -> [Interp b] -> m b
runInterp e dyns interps = case [f a | Interp f <- interps, Just a <- map fromDynamic dyns] of
                            []  -> fail $ "Does not type-check: " ++ unparseExprH e ++ "\n"
                            b:_ -> return b

-- | An 'Interp' @a@ is a /possible/ means of converting a 'Typeable' value to a value of type @a@.
data Interp :: * -> * where
   Interp :: Typeable a => (a -> b) -> Interp b

-- | The primitive way of building an 'Interp'.
interp :: Typeable a => (a -> b) -> Interp b
interp = Interp

instance Functor Interp where
  fmap :: (a -> b) -> Interp a -> Interp b
  fmap f (Interp g) = Interp (f . g)


interpExpr :: MonadState CommandLineState m => ExprH -> m [Dynamic]
interpExpr = interpExpr' False

-- input: list length n, each elem is a variable length list of possible interpretations
-- output: variable length list, each elem is list of length n
fromDynList :: [[Dynamic]] -> [[Dynamic]]
fromDynList [] = [[]]
fromDynList (hs:dynss) = [ h:t | h <- hs, t <- fromDynList dynss ]

toBoxedList :: (Extern a, Typeable b) => [[Dynamic]] -> ([a] -> b) -> [Dynamic]
toBoxedList dyns boxCon = [ toDyn $ boxCon (map unbox l) | dl <- dyns, Just l <- [mapM fromDynamic dl] ]

interpExpr' :: MonadState CommandLineState m => Bool -> ExprH -> m [Dynamic]
interpExpr' _   (SrcName str) = return [ toDyn $ StringBox str ]
interpExpr' _   (CoreH str)   = return [ toDyn $ CoreBox (CoreString str) ]
interpExpr' _   (ListH exprs) = do 
    dyns <- liftM fromDynList $ mapM (interpExpr' True) exprs
    return $    toBoxedList dyns StringListBox
             ++ toBoxedList dyns (PathBox . pathToSnocPath)
                -- ugly hack.  The whole dynamic stuff could do with overhauling.
             ++ toBoxedList dyns (TranslateCorePathBox . return . pathToSnocPath) 
             ++ toBoxedList dyns IntListBox
             ++ toBoxedList dyns RewriteCoreListBox

interpExpr' rhs (CmdName str) 
    | all isDigit str = do
        let i = read str
        return [ -- An Int is either a Path, or will be interpreted specially later.
                 toDyn $ IntBox i
                 -- TODO: Find a better long-term solution.
               , toDyn $ TranslateCorePathBox (deprecatedIntToPathT i) 
               ]
    | otherwise = do
        dict <- gets cl_dict
        case M.lookup str dict of
            Just dyns           -> do
                dyns' <- mapM provideState dyns
                return $ if rhs then toDyn (StringBox str) : dyns' else dyns'
            -- not a command, try as a string arg... worst case: dynApply fails with "bad type of expression"
            -- best case: 'help ls' works instead of 'help "ls"'. this is likewise done in the clause above
            Nothing | rhs       -> return [toDyn $ StringBox str]
                    | otherwise -> fail $ "User error, unrecognised HERMIT command: " ++ show str
interpExpr' _ (AppH e1 e2) = liftM2 dynCrossApply (interpExpr' False e1) (interpExpr' True e2)

-- We essentially treat externals of the type 'CommandLineState -> b' specially,
-- providing them the shell state here, so they don't need a monadic return type
-- in order to access it themselves.
provideState :: MonadState CommandLineState m => Dynamic -> m Dynamic
provideState dyn = do
    st <- get
    case dynApply dyn (toDyn $ box st) of
        Just d  -> return d
        Nothing -> return dyn

-- Cross product of possible applications.
dynCrossApply :: [Dynamic] -> [Dynamic] -> [Dynamic]
dynCrossApply fs xs = [ r | f <- fs, x <- xs, Just r <- return (dynApply f x)]

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