{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE RecordWildCards       #-}
{-# LANGUAGE TypeSynonymInstances  #-}
{-# LANGUAGE ViewPatterns          #-}
{-# OPTIONS -Wall -fno-warn-name-shadowing -fno-warn-orphans #-}

-- | The Haskell→Javascript compiler.

module Language.Fay

  where

import Language.Fay.Print              ()
import Language.Fay.Types

import Control.Applicative
import Control.Monad.Error
import Control.Monad.IO
import Control.Monad.State
import Data.List
import Data.Maybe
import Data.String
import Language.Haskell.Exts
import System.Process.Extra

--------------------------------------------------------------------------------
-- Top level entry points

-- | Compile something that compiles to something else.
compile :: CompilesTo from to => CompileConfig -> from -> IO (Either CompileError (to,CompileState))
compile config = runCompile config . compileTo

-- | Run the compiler.
runCompile :: CompileConfig -> Compile a -> IO (Either CompileError (a,CompileState))
runCompile config m = runErrorT (runStateT (unCompile m) state) where
  state = CompileState { stateConfig  = config
                       , stateExports = []
                       , stateModuleName = "Main"
                       , stateExportAll = True
                       }

-- | Compile a Haskell source string to a JavaScript source string.
compileViaStr :: (Show from,Show to,CompilesTo from to)
              => CompileConfig
              -> (from -> Compile to)
              -> String
              -> IO (Either CompileError (String,CompileState))
compileViaStr config with from =
  runCompile config
             (parseResult (throwError . uncurry ParseError)
                          (fmap printJS . with)
                          (parse from))

-- | Compile a Haskell source string to a JavaScript source string.
compileToAst :: (Show from,Show to,CompilesTo from to)
              => CompileConfig
              -> (from -> Compile to)
              -> String
              -> IO (Either CompileError (to,CompileState))
compileToAst config with from =
  runCompile config
             (parseResult (throwError . uncurry ParseError)
                          with
                          (parse from))

-- | Compile from a string.
compileFromStr :: (Parseable a, MonadError CompileError m) => (a -> m a1) -> String -> m a1
compileFromStr with from =
  parseResult (throwError . uncurry ParseError)
              (with)
              (parse from)

printCompile :: (Show from,Show to,CompilesTo from to)
              => CompileConfig
              -> (from -> Compile to)
              -> String
              -> IO ()
printCompile config with from = do
  result <- compileViaStr config with from
  case result of
    Left err -> putStrLn $ show err
    Right (ok,_) -> do writeFile "/tmp/x.js" ok
                       prettyPrintFile "/tmp/x.js" >>= putStr

--------------------------------------------------------------------------------
-- Compilers

-- | Compile Haskell module.
compileModule :: Module -> Compile [JsStmt]
compileModule (Module _ modulename _pragmas Nothing exports imports decls) = do
  modify $ \s -> s { stateModuleName = modulename
                   , stateExportAll = isNothing exports
                   }
  mapM_ emitExport (fromMaybe [] exports)
  imported <- fmap concat (mapM compileImport imports)
  current <- compileDecls True decls
  return (imported ++ current)
compileModule mod = throwError (UnsupportedModuleSyntax mod)

instance CompilesTo Module [JsStmt] where compileTo = compileModule

-- | Compile the given import.
compileImport :: ImportDecl -> Compile [JsStmt]
compileImport (ImportDecl _ (ModuleName name) _ _ _ _ _)
  | isPrefixOf "Language.Fay." name || name == "Prelude" = return []
compileImport (ImportDecl _ (ModuleName name) False _ Nothing Nothing Nothing) = do
  contents <- io (readFile (replace '.' '/' name ++ ".hs"))
  compileFromStr compileModule contents
    where replace c r = map (\x -> if x == c then r else x)
compileImport i =
  error $ "Import syntax not supported. " ++
        "The compiler writer was too lazy to support that.\n" ++
        "It was: " ++ show i

-- | Compile Haskell declaration.
compileDecls :: Bool -> [Decl] -> Compile [JsStmt]
compileDecls toplevel decls = do
  case decls of
    [] -> return []
    (TypeSig _ _ sig:bind@PatBind{}:decls) -> appendM (compilePatBind toplevel (Just sig) bind)
                                                      (compileDecls toplevel decls)
    (decl:decls) -> appendM (compileDecl toplevel decl)
                            (compileDecls toplevel decls)

  where appendM m n = do x <- m
                         xs <- n
                         return (x ++ xs)

-- | Compile a declaration.
compileDecl :: Bool -> Decl -> Compile [JsStmt]
compileDecl toplevel decl =
  case decl of
    pat@PatBind{} -> compilePatBind toplevel Nothing pat
    FunBind matches -> compileFunCase toplevel matches
    DataDecl _ DataType _ _ _ constructors _ -> compileDataDecl toplevel decl constructors
    -- Just ignore type aliases and signatures.
    TypeDecl{} -> return []
    TypeSig{} -> return []
    InfixDecl{} -> return []
    ClassDecl{} -> return []
    InstDecl{} -> return [] -- FIXME: Ignore.
    _ -> throwError (UnsupportedDeclaration decl)

-- | Compile a top-level pattern bind.
compilePatBind :: Bool -> Maybe Type -> Decl -> Compile [JsStmt]
compilePatBind toplevel sig pat = do
  case pat of
    PatBind _ (PVar ident) Nothing (UnGuardedRhs rhs) (BDecls []) ->
      case ffiExp rhs <|> ffiProp rhs of
        Just detail@(binding,_,_) ->
          case sig of
            Nothing -> compileNormalPatBind toplevel ident rhs
            Just sig -> case () of
              () | func binding   -> compileFFIFunc sig ident detail
                 | method binding -> compileFFIMethod sig ident detail
                 | setprop binding -> compileFFISetProp sig ident detail
                 | otherwise      -> throwError (FfiNeedsTypeSig pat)
        _ -> compileNormalPatBind toplevel ident rhs
    _ -> throwError (UnsupportedDeclaration pat)

  where func = flip elem ["foreignFay","foreignPure","foreignValue"]
        method = flip elem ["foreignMethodFay","foreignProp","foreignPropFay","foreignMethod"]
        setprop = flip elem ["foreignSetProp"]

        ffiExp (App (App (Var (UnQual (Ident ident)))
                         (Lit (String name)))
                    (Con (UnQual (Ident (reads -> [(typ,"")])))))
          = Just (ident,name,typ)
        ffiExp _ = Nothing

        ffiProp (App (Var (UnQual (Ident ident)))
                     (Lit (String name)))
          = Just (ident,name,FayNone)
        ffiProp _ = Nothing

-- | Compile a normal simple pattern binding.
compileNormalPatBind :: Bool -> Name -> Exp -> Compile [JsStmt]
compileNormalPatBind toplevel ident rhs = do
  body <- compileExp rhs
  bind <- bindToplevel toplevel (UnQual ident) (thunk body)
  return [bind]

-- | Compile a foreign function.
compileFFIFunc :: Type -> Name -> (String,String,FayReturnType) -> Compile [JsStmt]
compileFFIFunc sig ident detail@(_,name,_) = do
  let args = zipWith const uniqueNames [1..typeArity sig]
  compileFFI sig ident detail (JsRawName name) args args

-- | Compile a foreign method.
compileFFIMethod :: Type -> Name -> (String,String,FayReturnType) -> Compile [JsStmt]
compileFFIMethod sig ident detail@(_,name,_) = do
  let args = zipWith const uniqueNames [1..typeArity sig]
      jsargs = drop 1 args
      obj = head args
  compileFFI sig ident detail (JsGetPropExtern (force (JsName obj)) (fromString name)) args jsargs

-- | Compile a foreign method.
compileFFISetProp :: Type -> Name -> (String,String,FayReturnType) -> Compile [JsStmt]
compileFFISetProp sig ident detail@(_,name,_) = do
  let args = zipWith const uniqueNames [1..typeArity sig]
      jsargs = drop 1 args
      obj = head args
  compileFFI sig
             ident
             detail
             (JsUpdatePropExtern (force (JsName obj))
                                 (fromString name)
                                 (serialize (head (tail funcTypes))
                                            (JsName (head jsargs))))
             args
             []
               
  where funcTypes = functionTypeArgs sig

-- | Compile an FFI call.
compileFFI :: Type
           -> Name
           -> (String,String,FayReturnType)
           -> JsExp
           -> [JsName]
           -> [JsName]
           -> Compile [JsStmt]
compileFFI sig ident (binding,_,typ) exp params args = do
  let innerexp
        | length args == 0 && elem binding ["foreignProp","foreignPropFay","foreignValue"] = exp
        | binding == "foreignSetProp" = exp
        | otherwise = JsApp exp
                            (map (\(typ,name) -> serialize typ (JsName name))
                                 (zip types args))
  bind <- bindToplevel True
                       (UnQual ident)
                       (foldr (\name inner -> JsFun [name] [] (Just inner))
                              (thunk
                               (maybeMonad
                                (if binding == "foreignSetProp"
                                    then innerexp
                                    else unserialize typ innerexp)))
                              params)
  return [bind]

  where (maybeMonad,types) | binding == "foreignFay"       = (monad,funcTypes)
                           | binding == "foreignProp"      = (id,drop 1 funcTypes)
                           | binding == "foreignMethodFay" = (monad,drop 1 funcTypes)
                           | binding == "foreignPropFay"   = (monad,drop 1 funcTypes)
                           | binding == "foreignMethod"    = (id,drop 1 funcTypes)
                           | binding == "foreignSetProp"   = (monad,[])
                           | otherwise                     = (id,funcTypes)
        funcTypes = functionTypeArgs sig

-- | These are the data types that are serializable directly to native
-- JS data types. Strings, floating points and arrays. The others are:
-- actiosn in the JS monad, which are thunks that shouldn't be forced
-- when serialized but wrapped up as JS zero-arg functions, and
-- unknown types can't be converted but should at least be forced.
data ArgType = FunctionType | JsType | StringType | DoubleType | ListType | BoolType | UnknownType
  deriving (Show,Eq)

-- | Serialize a value to native JS, if possible.
serialize :: ArgType -> JsExp -> JsExp
serialize typ exp =
  JsApp (JsName (hjIdent "serialize"))
        [JsName (fromString (show typ)),exp]

-- | Get arg types of a function type.
functionTypeArgs :: Type -> [ArgType]
functionTypeArgs t =
  case t of
    TyForall _ _ i -> functionTypeArgs i
    TyFun a b      -> argType a : functionTypeArgs b
    TyParen st     -> functionTypeArgs st
    _              -> []

  where argType t =
          case t of
            TyApp (TyCon "Fay") _ -> JsType
            TyCon "String"       -> StringType
            TyCon "Double"       -> DoubleType
            TyCon "Bool"         -> BoolType
            TyFun{}              -> FunctionType
            TyList _             -> ListType
            _                    -> UnknownType

-- | Get the arity of a type.
typeArity :: Type -> Integer
typeArity t =
  case t of
    TyForall _ _ i -> typeArity i
    TyFun _ b      -> 1 + typeArity b
    TyParen st     -> typeArity st
    _              -> 0

-- | Compile a data declaration.
compileDataDecl :: Bool -> Decl -> [QualConDecl] -> Compile [JsStmt]
compileDataDecl toplevel decl constructors = do
  fmap concat $
    forM constructors $ \(QualConDecl _ _ _ condecl) ->
      case condecl of
        ConDecl (UnQual -> name) types  -> fmap return (makeDataCons name types [])
        RecDecl (UnQual -> name) fields -> do
          cons <- makeDataCons name (map snd fields) (map fst fields)
          funs <- makeAccessors (zip [1..] (map fst fields))
          return (cons : funs)
        _ -> throwError (UnsupportedDeclaration decl)

  where makeDataCons name types fields = do
          let slots = (map (fromString . ("slot"++) . show . fst)
                           (zip [1 :: Integer ..] types))
          return $
            JsVar name
                  (foldr (\slot inner -> JsFun [slot] [] (Just inner))
                         (thunk (JsList ((JsNew (hjIdent "Constructor")
                                                (JsLit (JsStr (qname name)) :
                                                 concat (map (map (JsLit . JsStr . unname)) fields)))
                                         : map JsName slots)))
                         slots)
        makeAccessors fields = do
          fmap concat $
            forM fields $ \(i,field) ->
              forM field $ \name ->
                bindToplevel toplevel
                             (UnQual name)
                             (JsFun ["x"]
                                    []
                                    (Just (thunk (JsIndex i (force (JsName "x"))))))

-- | Extract the string from a qname.
qname :: QName -> String
qname (UnQual (Ident str)) = str
qname _ = error "qname: Expected unqualified ident." -- FIXME:

-- | Extra the string from an ident.
unname :: Name -> String
unname (Ident str) = str
unname _ = error "Expected ident from uname." -- FIXME:

-- | Compile a function which pattern matches (causing a case analysis).
compileFunCase :: Bool -> [Match] -> Compile [JsStmt]
compileFunCase _toplevel [] = return []
compileFunCase toplevel matches@(Match _ name argslen _ _ _:_) = do
  tco <- config configTCO
  pats <- fmap optimizePatConditions $ forM matches $ \match@(Match _ _ pats _ rhs wheres) -> do
    unless (noBinds wheres) $ do _ <- throwError (UnsupportedWhereInMatch match) -- TODO: Support `where'.
                                 return ()
    exp <- compileRhs rhs
    foldM (\inner (arg,pat) -> do
             compilePat (JsName arg) pat inner)
          [JsEarlyReturn exp]
          (zip args pats)
  bind <- bindToplevel toplevel
                       (UnQual name)
                       (foldr (\arg inner -> JsFun [arg] [] (Just inner))
                              (stmtsThunk (let stmts = (concat pats ++ basecase)
                                           in if tco
                                                 then optimizeTailCalls args name stmts
                                                 else stmts))
                              args)
  return [bind]
  where args = zipWith const uniqueNames argslen
        basecase = if any isWildCardMatch matches
                      then []
                      else [throw ("unhandled case in " ++ show name)
                                  (JsList (map JsName args))]
        isWildCardMatch (Match _ _ pats _ _ _) = all isWildCardPat pats
        noBinds (BDecls []) = True
        noBinds (IPBinds []) = True
        noBinds _ = False

-- | Optimize functions in tail-call form.
optimizeTailCalls :: [JsParam] -- ^ The function parameters.
                  -> Name      -- ^ The function name.
                  -> [JsStmt]  -- ^ The body of the function.
                  -> [JsStmt]  -- ^ A new optimized function body.
optimizeTailCalls params name stmts = abandonIfNoChange $
  JsWhile (JsLit (JsBool True))
          (concatMap replaceTailStmt
                     (reverse (zip (reverse stmts) [0::Integer ..])))
  
  where replaceTailStmt (JsIf cond sothen orelse,i) = [JsIf cond (concatMap (replaceTailStmt . (,i)) sothen)
                                                                 (concatMap (replaceTailStmt . (,i)) orelse)]
        replaceTailStmt (JsEarlyReturn exp,i) = expTailReplace i exp
        replaceTailStmt (x,_) = [x]
        expTailReplace i (flatten -> Just (JsName (UnQual call):args@(_:_)))
          | call == name = updateParamsInstead i args
        expTailReplace _i original = [JsEarlyReturn original]
        updateParamsInstead i args = zipWith JsUpdate params args ++
                                     [JsContinue | i /= 0]
        abandonIfNoChange (JsWhile _ newstmts)
          | newstmts == stmts = stmts
        abandonIfNoChange new = [new]

-- | Flatten an application expression into function : arg : arg : []
flatten :: JsExp -> Maybe [JsExp]
flatten (JsApp op@JsApp{} arg) = do
  inner <- expand op
  return (inner ++ arg)
flatten name@JsName{} = return [name]
flatten _ = Nothing

-- | Expand a forced value into the value.
expand :: JsExp -> Maybe [JsExp]
expand (JsApp (JsName (UnQual (Ident "_"))) xs) = do
  fmap concat (mapM flatten xs)
expand _ = Nothing

-- -- | Run a JS file.
prettyPrintFile :: String -> IO String
prettyPrintFile file = fmap (either id id) (readAllFromProcess "js-beautify" file)

-- | Compile a right-hand-side expression.
compileRhs :: Rhs -> Compile JsExp
compileRhs (UnGuardedRhs exp) = compileExp exp
compileRhs rhs                = throwError (UnsupportedRhs rhs)

-- | Compile a pattern match binding.
compileFunMatch :: Bool -> Match -> Compile [JsStmt]
compileFunMatch toplevel match =
  case match of
    (Match _ name args Nothing (UnGuardedRhs rhs) _) -> do
      body <- compileExp rhs
      args <- mapM patToArg args
      bind <- bindToplevel toplevel
                           (UnQual name)
                           (foldr (\arg inner -> JsFun [arg] [] (Just inner))
                                  (thunk body)
                                  args)
      return [bind]
    match -> throwError (UnsupportedMatchSyntax match)

  where patToArg (PVar name) = return (UnQual name)
        patToArg _           = throwError (UnsupportedMatchSyntax match)

instance CompilesTo Decl [JsStmt] where compileTo = compileDecl False

-- | Compile Haskell expression.
compileExp :: Exp -> Compile JsExp
compileExp exp =
  case exp of
    Paren exp                     -> compileExp exp
    Var (UnQual (Ident "return")) -> return (JsName (hjIdent "return"))
    Var qname                     -> return (JsName qname)
    Lit lit                       -> compileLit lit
    App exp1 exp2                 -> compileApp exp1 exp2
    InfixApp exp1 op exp2         -> compileInfixApp exp1 op exp2
    Let (BDecls decls) exp        -> compileLet decls exp
    List []                       -> return JsNull
    List xs                       -> compileList xs
    Tuple xs                      -> compileList xs
    If cond conseq alt            -> compileIf cond conseq alt
    Case exp alts                 -> compileCase exp alts
    Con (UnQual (Ident "True"))   -> return (JsName "true")
    Con (UnQual (Ident "False"))  -> return (JsName "false")
    Con exp                       -> return (JsName exp)
    Do stmts                      -> compileDoBlock stmts
    Lambda _ pats exp             -> compileLambda pats exp
    EnumFrom i                    -> do e <- compileExp i
                                        return (JsApp (JsName "enumFrom") [e])
    EnumFromTo i i'               -> do f <- compileExp i
                                        t <- compileExp i'
                                        return (JsApp (JsApp (JsName "enumFromTo") [f])
                                                      [t])
    ExpTypeSig _ e _ -> compileExp e

    exp -> throwError (UnsupportedExpression exp)

instance CompilesTo Exp JsExp where compileTo = compileExp

-- | Compile simple application.
compileApp :: Exp -> Exp -> Compile JsExp
compileApp exp1 exp2 = do
   flattenApps <- config configFlattenApps
   if flattenApps then method2 else method1
   where
  -- Method 1:           
  -- In this approach code ends up looking like this:
  -- a(a(a(a(a(a(a(a(a(a(L)(c))(b))(0))(0))(y))(t))(a(a(F)(3*a(a(d)+a(a(f)/20))))*a(a(f)/2)))(140+a(f)))(y))(t)})
  -- Which might be OK for speed, but increases the JS stack a fair bit.
  method1 =
    JsApp <$> (forceFlatName <$> compileExp exp1)
          <*> fmap return (compileExp exp2)
  forceFlatName name = JsApp (JsName "_") [name]

  -- Method 2:
  -- In this approach code ends up looking like this:
  -- d(O,a,b,0,0,B,w,e(d(I,3*e(e(c)+e(e(g)/20))))*e(e(g)/2),140+e(g),B,w)}),d(K,g,e(c)+0.05))
  -- Which should be much better for the stack and readability, but probably not great for speed.
  method2 = fmap flatten $
    JsApp <$> compileExp exp1
          <*> fmap return (compileExp exp2)
  flatten (JsApp op args) =
   case op of
     JsApp l r -> JsApp l (r ++ args)
     _        -> JsApp (JsName "__") (op : args)
  flatten x = x

-- | Compile an infix application, optimizing the JS cases.
compileInfixApp :: Exp -> QOp -> Exp -> Compile JsExp
compileInfixApp exp1 op exp2 = do
  config <- config id
  case getOp op of
    UnQual (Symbol symbol)
      | symbol `elem` words "* + - / < > || &&" -> do
          e1 <- compileExp exp1
          e2 <- compileExp exp2
          return (JsInfix symbol (forceInlinable config e1) (forceInlinable config e2)) 
    _ -> do
      var <- resolveOpToVar op
      compileExp (App (App var exp1) exp2)

  where getOp (QVarOp op) = op
        getOp (QConOp op) = op
  
-- | Compile a list expression.
compileList :: [Exp] -> Compile JsExp
compileList xs = do
  exps <- mapM compileExp xs
  return (JsApp (JsName (hjIdent "list")) [JsList exps])

-- | Compile an if.
compileIf :: Exp -> Exp -> Exp -> Compile JsExp
compileIf cond conseq alt =
  JsTernaryIf <$> fmap force (compileExp cond)
              <*> compileExp conseq
              <*> compileExp alt

-- | Compile a lambda.
compileLambda :: [Pat] -> Exp -> Compile JsExp
compileLambda pats exp = do
  exp <- compileExp exp
  stmts <- foldM (\inner (param,pat) -> do
                   stmts <- compilePat (JsName param) pat inner
                   return [JsEarlyReturn (JsFun [param] (stmts ++ [unhandledcase param | not allfree]) Nothing)])
                 [JsEarlyReturn exp]
                 (reverse (zip uniqueNames pats))
  case stmts of
    [JsEarlyReturn fun@JsFun{}] -> return fun
    _ -> error "Unexpected statements in compileLambda"

  where unhandledcase = throw "unhandled case" . JsName
        allfree = all isWildCardPat pats

-- | Compile case expressions.
compileCase :: Exp -> [Alt] -> Compile JsExp
compileCase exp alts = do
  exp <- compileExp exp
  pats <- fmap optimizePatConditions $ mapM (compilePatAlt (JsName (tmpName exp))) alts
  return $
    (JsApp (JsFun [tmpName exp]
                  (concat pats)
                  (if any isWildCardAlt alts
                      then Nothing
                      else Just (throwExp "unhandled case" (JsName (tmpName exp)))))
           [exp])

-- | Compile a do block.
compileDoBlock :: [Stmt] -> Compile JsExp
compileDoBlock stmts = do
  doblock <- foldM compileStmt Nothing (reverse stmts)
  maybe (throwError EmptyDoBlock) compileExp doblock

-- | Compile a statement of a do block.
compileStmt :: Maybe Exp -> Stmt -> Compile (Maybe Exp)
compileStmt inner stmt =
  case inner of
    Nothing -> initStmt
    Just inner -> subsequentStmt inner

  where initStmt =
          case stmt of
            Qualifier exp -> return (Just exp)
            LetStmt{}     -> throwError LetUnsupported
            _             -> throwError InvalidDoBlock

        subsequentStmt inner =
          case stmt of
            Generator loc pat exp -> compileGenerator loc pat inner exp
            Qualifier exp -> return (Just (InfixApp exp
                                                    (QVarOp (UnQual (Symbol ">>")))
                                                    inner))
            LetStmt{} -> throwError LetUnsupported
            RecStmt{} -> throwError RecursiveDoUnsupported

        compileGenerator srcloc pat inner exp = do
          let body = (Lambda srcloc [pat] inner)
          return (Just (InfixApp exp
                                 (QVarOp (UnQual (Symbol ">>=")))
                                 body))

-- | Compile the given pattern against the given expression.
compilePatAlt :: JsExp -> Alt -> Compile [JsStmt]
compilePatAlt exp (Alt _ pat rhs _) = do
  alt <- compileGuardedAlt rhs
  compilePat exp pat [JsEarlyReturn alt]

-- | Compile the given pattern against the given expression.
compilePat :: JsExp -> Pat -> [JsStmt] -> Compile [JsStmt]
compilePat exp pat body = do
  case pat of
    PVar name       -> return ([JsVar (UnQual name) exp] ++ body)
    PApp cons pats  -> compilePApp cons pats exp body
    PLit literal    -> compilePLit exp literal body
    PParen pat      -> compilePat exp pat body
    PWildCard       -> return body
    pat@PInfixApp{} -> compileInfixPat exp pat body
    PList pats      -> compilePList pats body exp
    PTuple pats     -> compilePList pats body exp
    pat             -> throwError (UnsupportedPattern pat)

-- | Compile a literal value from a pattern match.
compilePLit :: JsExp -> Literal -> [JsStmt] -> Compile [JsStmt]
compilePLit exp literal body = do
  lit <- compileLit literal
  return [JsIf (equalExps exp lit)
               body
               []]

-- | Equality test for two expressions, with some optimizations.
equalExps :: JsExp -> JsExp -> JsExp
equalExps a b 
  | isConstant a && isConstant b = JsEq a b
  | isConstant a = JsEq a (force b)
  | isConstant b = JsEq (force a) b
  | otherwise =
     JsApp (JsName (hjIdent "equal")) [a,b]

-- | Is a JS expression a literal (constant)?
isConstant :: JsExp -> Bool
isConstant JsLit{} = True
isConstant _       = False

-- | Compile a pattern application.
compilePApp :: QName -> [Pat] -> JsExp -> [JsStmt] -> Compile [JsStmt]
compilePApp cons pats exp body = do
  let forcedExp = force exp
  substmts <- foldM (\body (i,pat) -> compilePat (JsIndex i forcedExp) pat body)
                    body
                    (reverse (zip [1..] pats))
  let constructor = JsIndex 0 forcedExp
      compareConstructorNames
        -- Special-casing on the booleans.
        | cons == "True" = JsEq forcedExp (JsLit (JsBool True))
        | cons == "False" = JsEq forcedExp (JsLit (JsBool False))
        -- Everything else, generic:
        | otherwise =
            JsEq (JsGetProp constructor "name")
                 (JsLit (JsStr (qname cons)))
  return [JsIf compareConstructorNames
               substmts
               []]

-- | Compile a pattern list.
compilePList :: [Pat] -> [JsStmt] -> JsExp -> Compile [JsStmt]
compilePList [] body exp =
  return [JsIf (JsEq (force exp) JsNull) body []]
compilePList pats body exp = do
  let forcedExp = force exp
  substmts <- foldM (\body (i,pat) -> compilePat (JsApp (JsApp (JsName (hjIdent "index"))
                                                               [JsLit (JsInt i)])
                                                        [forcedExp])
                                                 pat body)
                    body
                    (reverse (zip [0..] pats))
  return substmts

-- | Compile an infix pattern (e.g. cons and tuples.)
compileInfixPat :: JsExp -> Pat -> [JsStmt] -> Compile [JsStmt]
compileInfixPat exp pat@(PInfixApp left (Special cons) right) body =
  case cons of
    Cons -> do
      let forcedExp = JsName (tmpName exp)
          x = (JsGetProp forcedExp "car")
          xs = (JsGetProp forcedExp "cdr")
      rightMatch <- compilePat xs right body
      leftMatch <- compilePat x left rightMatch
      return [JsVar (tmpName exp) (force exp)
             ,JsIf (JsInstanceOf forcedExp (hjIdent "Cons"))
                   leftMatch
                   []]
    _ -> throwError (UnsupportedPattern pat)
compileInfixPat _ pat _ = throwError (UnsupportedPattern pat)

-- | Compile a guarded alt.
compileGuardedAlt :: GuardedAlts -> Compile JsExp
compileGuardedAlt alt =
  case alt of
    UnGuardedAlt exp -> compileExp exp
    alt -> throwError (UnsupportedGuardedAlts alt)

-- | Compile a let expression.
compileLet :: [Decl] -> Exp -> Compile JsExp
compileLet decls exp = do
  body <- compileExp exp
  binds <- mapM compileLetDecl decls
  return (JsApp (JsFun [] (concat binds) (Just body)) [])

-- | Compile let declaration.
compileLetDecl :: Decl -> Compile [JsStmt]
compileLetDecl decl =
  case decl of
    decl@PatBind{} -> compileDecls False [decl]
    decl@FunBind{} -> compileDecls False [decl]
    _              -> throwError (UnsupportedLetBinding decl)

-- | Compile Haskell literal.
compileLit :: Literal -> Compile JsExp
compileLit lit =
  case lit of
    Char ch       -> return (JsLit (JsChar ch))
    Int integer   -> return (JsLit (JsInt (fromIntegral integer))) -- FIXME:
    Frac rational -> return (JsLit (JsFloating (fromRational rational)))
    -- TODO: Use real JS strings instead of array, probably it will
    -- lead to the same result.
    String string -> return (JsApp (JsName (hjIdent "list"))
                                   [JsLit (JsStr string)])
    lit           -> throwError (UnsupportedLiteral lit)

--------------------------------------------------------------------------------
-- Compilation utilities

-- | Generate unique names.
uniqueNames :: [JsParam]
uniqueNames = map (fromString . ("$_" ++))
            $ map return "abcxyz" ++
              zipWith (:) (cycle "v")
                          (map show [1 :: Integer ..])

-- | Optimize pattern matching conditions by merging conditions in common.
optimizePatConditions :: [[JsStmt]] -> [[JsStmt]]
optimizePatConditions = concat . map merge . groupBy sameIf where
  sameIf [JsIf cond1 _ _] [JsIf cond2 _ _] = cond1 == cond2
  sameIf _ _ = False
  merge xs@([JsIf cond _ _]:_) =
    [[JsIf cond (concat (optimizePatConditions (map getIfConsequent xs))) []]]
  merge noifs = noifs
  getIfConsequent [JsIf _ cons _] = cons
  getIfConsequent other = other

-- | Throw a JS exception.
throw :: String -> JsExp -> JsStmt
throw msg exp = JsThrow (JsList [JsLit (JsStr msg),exp])

-- | Throw a JS exception (in an expression).
throwExp :: String -> JsExp -> JsExp
throwExp msg exp = JsThrowExp (JsList [JsLit (JsStr msg),exp])

-- | Is an alt a wildcard?
isWildCardAlt :: Alt -> Bool
isWildCardAlt (Alt _ pat _ _) = isWildCardPat pat

-- | Is a pattern a wildcard?
isWildCardPat :: Pat -> Bool
isWildCardPat PWildCard{} = True
isWildCardPat PVar{}      = True
isWildCardPat _           = False

-- | A temporary name for testing conditions and such.
tmpName :: JsExp -> JsName
tmpName exp =
  fromString $
    case exp of
      JsName (qname -> x) -> "$_" ++ x
      _ -> ":tmp"

-- | Wrap an expression in a thunk.
thunk :: JsExp -> JsExp
-- thunk exp = JsNew (hjIdent "Thunk") [JsFun [] [] (Just exp)]
thunk exp =
  case exp of
    -- JS constants don't need to be in thunks, they're already strict.
    JsLit{} -> exp
    JsName "true" -> exp
    JsName "false" -> exp
    -- Functions (e.g. lets) used for introducing a new lexical scope
    -- aren't necessary inside a thunk. This is a simple aesthetic
    -- optimization.
    JsApp fun@JsFun{} [] -> JsNew ":thunk" [fun]
    -- Otherwise make a regular thunk.
    _ -> JsNew ":thunk" [JsFun [] [] (Just exp)]

-- | Wrap an expression in a thunk.
monad :: JsExp -> JsExp
monad exp = JsNew (hjIdent "Monad") [exp]

-- | Wrap an expression in a thunk.
stmtsThunk :: [JsStmt] -> JsExp
stmtsThunk stmts = JsNew ":thunk" [JsFun [] stmts Nothing]

unserialize :: FayReturnType -> JsExp -> JsExp
unserialize typ exp =
  JsApp (JsName (hjIdent "unserialize"))
        [JsLit (JsStr (showReturnType typ)),exp]
    
  where showReturnType typ =
          case typ of
            FayArray -> "array"
            FayList -> "list"
            FayString -> "string"
            FayBool -> "bool"
            FayNone -> ""

-- | Force an expression in a thunk.
force :: JsExp -> JsExp
force exp
  | isConstant exp = exp
  | otherwise = JsApp (JsName "_") [exp]

-- | Force an expression in a thunk.
forceInlinable :: CompileConfig -> JsExp -> JsExp
forceInlinable config exp
  | isConstant exp = exp
  | configInlineForce config =
    JsParen (JsTernaryIf (exp `JsInstanceOf` ":thunk")
                         (JsApp (JsName "_") [exp])
                         exp)
  | otherwise = JsApp (JsName "_") [exp]

-- | Resolve operators to only built-in (for now) functions.
resolveOpToVar :: QOp -> Compile Exp
resolveOpToVar op =
  case getOp op of
    UnQual (Symbol symbol)
      | symbol == "*"   -> return (Var (hjIdent "mult"))
      | symbol == "+"   -> return (Var (hjIdent "add"))
      | symbol == "-"   -> return (Var (hjIdent "sub"))
      | symbol == "/"   -> return (Var (hjIdent "div"))
      | symbol == "=="  -> return (Var (hjIdent "eq"))
      | symbol == "/="  -> return (Var (hjIdent "neq"))
      | symbol == ">"   -> return (Var (hjIdent "gt"))
      | symbol == "<"   -> return (Var (hjIdent "lt"))
      | symbol == ">="  -> return (Var (hjIdent "gte"))
      | symbol == "<="  -> return (Var (hjIdent "lte"))
      | symbol == "&&"  -> return (Var (hjIdent "and"))
      | symbol == "||"  -> return (Var (hjIdent "or"))
      | symbol == ">>=" -> return (Var (hjIdent "bind"))
      | symbol == ">>"  -> return (Var (hjIdent "then"))
      | otherwise       -> return (Var (fromString symbol))
    Special Cons        -> return (Var (hjIdent "cons"))
    _                   -> throwError (UnsupportedOperator op)

  where getOp (QVarOp op) = op
        getOp (QConOp op) = op

-- | Make an identifier from the built-in HJ module.
hjIdent :: String -> QName
hjIdent = Qual (ModuleName "Fay") . Ident

-- | Make a top-level binding.
bindToplevel :: Bool -> QName -> JsExp -> Compile JsStmt
bindToplevel toplevel name exp = do
  exportAll <- gets stateExportAll
  when (toplevel && exportAll) $ emitExport (EVar name)
  return (JsVar name exp)

-- | Emit exported names.
emitExport :: ExportSpec -> Compile ()
emitExport spec = 
  case spec of
    EVar (UnQual name) -> modify $ \s -> s { stateExports = name : stateExports s }
    EVar _             -> error "Emitted a qualifed export, not supported."
    _ -> throwError (UnsupportedExportSpec spec)

--------------------------------------------------------------------------------
-- Utilities

-- | Parse result.
parseResult :: ((SrcLoc,String) -> b) -> (a -> b) -> ParseResult a -> b
parseResult fail ok result =
  case result of
    ParseOk a -> ok a
    ParseFailed srcloc msg -> fail (srcloc,msg)

-- | Get a config option.
config :: (CompileConfig -> a) -> Compile a
config f = gets (f . stateConfig)