GHCi/TH.hs

    {-# LANGUAGE ScopedTypeVariables, StandaloneDeriving, DeriveGeneric,
        TupleSections, RecordWildCards, InstanceSigs #-}
    {-# OPTIONS_GHC -fno-warn-name-shadowing #-}

    -- |
    -- Running TH splices
    --
    module GHCi.TH
      ( startTH
      , runModFinalizerRefs
      , runTH
      , GHCiQException(..)
      ) where

    {- Note [Remote Template Haskell]

    Here is an overview of how TH works with -fexternal-interpreter.

    Initialisation
    ~~~~~~~~~~~~~~

    GHC sends a StartTH message to the server (see TcSplice.getTHState):

       StartTH :: Message (RemoteRef (IORef QState))

    The server creates an initial QState object, makes an IORef to it, and
    returns a RemoteRef to this to GHC. (see GHCi.TH.startTH below).

    This happens once per module, the first time we need to run a TH
    splice.  The reference that GHC gets back is kept in
    tcg_th_remote_state in the TcGblEnv, and passed to each RunTH call
    that follows.

    For each splice
    ~~~~~~~~~~~~~~~

    1. GHC compiles a splice to byte code, and sends it to the server: in
       a CreateBCOs message:

       CreateBCOs :: [LB.ByteString] -> Message [HValueRef]

    2. The server creates the real byte-code objects in its heap, and
       returns HValueRefs to GHC.  HValueRef is the same as RemoteRef
       HValue.

    3. GHC sends a RunTH message to the server:

      RunTH
       :: RemoteRef (IORef QState)
            -- The state returned by StartTH in step1
       -> HValueRef
            -- The HValueRef we got in step 4, points to the code for the splice
       -> THResultType
            -- Tells us what kind of splice this is (decl, expr, type, etc.)
       -> Maybe TH.Loc
            -- Source location
       -> Message (QResult ByteString)
            -- Eventually it will return a QResult back to GHC.  The
            -- ByteString here is the (encoded) result of the splice.

    4. The server runs the splice code.

    5. Each time the splice code calls a method of the Quasi class, such
       as qReify, a message is sent from the server to GHC.  These
       messages are defined by the THMessage type.  GHC responds with the
       result of the request, e.g. in the case of qReify it would be the
       TH.Info for the requested entity.

    6. When the splice has been fully evaluated, the server sends
       RunTHDone back to GHC.  This tells GHC that the server has finished
       sending THMessages and will send the QResult next.

    8. The server then sends a QResult back to GHC, which is notionally
       the response to the original RunTH message.  The QResult indicates
       whether the splice succeeded, failed, or threw an exception.

    After typechecking
    ~~~~~~~~~~~~~~~~~~

    GHC sends a FinishTH message to the server (see TcSplice.finishTH).
    The server runs any finalizers that were added by addModuleFinalizer.

    Other Notes on TH / Remote GHCi

      * Note [Remote GHCi] in compiler/ghci/GHCi.hs
      * Note [External GHCi pointers] in compiler/ghci/GHCi.hs
      * Note [TH recover with -fexternal-interpreter] in
        compiler/typecheck/TcSplice.hs
    -}

    import GHCi.Message
    import GHCi.RemoteTypes
    import GHC.Serialized

    import Control.Exception
    import qualified Control.Monad.Fail as Fail
    import Data.Binary
    import Data.Binary.Put
    import Data.ByteString (ByteString)
    import qualified Data.ByteString as B
    import qualified Data.ByteString.Lazy as LB
    import Data.Data
    import Data.Dynamic
    import Data.IORef
    import Data.Map (Map)
    import qualified Data.Map as M
    import Data.Maybe
    import GHC.Desugar
    import qualified Language.Haskell.TH        as TH
    import qualified Language.Haskell.TH.Syntax as TH
    import Unsafe.Coerce

    -- | Create a new instance of 'QState'
    initQState :: Pipe -> QState
    initQState p = QState M.empty Nothing p

    -- | The monad in which we run TH computations on the server
    newtype GHCiQ a = GHCiQ { runGHCiQ :: QState -> IO (a, QState) }

    -- | The exception thrown by "fail" in the GHCiQ monad
    data GHCiQException = GHCiQException QState String
      deriving Show

    instance Exception GHCiQException

    instance Functor GHCiQ where
      fmap f (GHCiQ s) = GHCiQ $ fmap (\(x,s') -> (f x,s')) . s

    instance Applicative GHCiQ where
      f <*> a = GHCiQ $ \s ->
        do (f',s')  <- runGHCiQ f s
           (a',s'') <- runGHCiQ a s'
           return (f' a', s'')
      pure x = GHCiQ (\s -> return (x,s))

    instance Monad GHCiQ where
      m >>= f = GHCiQ $ \s ->
        do (m', s')  <- runGHCiQ m s
           (a,  s'') <- runGHCiQ (f m') s'
           return (a, s'')
      fail = Fail.fail

    instance Fail.MonadFail GHCiQ where
      fail err  = GHCiQ $ \s -> throwIO (GHCiQException s err)

    getState :: GHCiQ QState
    getState = GHCiQ $ \s -> return (s,s)

    noLoc :: TH.Loc
    noLoc = TH.Loc "<no file>" "<no package>" "<no module>" (0,0) (0,0)

    -- | Send a 'THMessage' to GHC and return the result.
    ghcCmd :: Binary a => THMessage (THResult a) -> GHCiQ a
    ghcCmd m = GHCiQ $ \s -> do
      r <- remoteTHCall (qsPipe s) m
      case r of
        THException str -> throwIO (GHCiQException s str)
        THComplete res -> return (res, s)

    instance TH.Quasi GHCiQ where
      qNewName str = ghcCmd (NewName str)
      qReport isError msg = ghcCmd (Report isError msg)

      -- See Note [TH recover with -fexternal-interpreter] in TcSplice
      qRecover (GHCiQ h) (GHCiQ a) = GHCiQ $ \s -> (do
        remoteTHCall (qsPipe s) StartRecover
        (r, s') <- a s
        remoteTHCall (qsPipe s) (EndRecover False)
        return (r,s'))
          `catch`
           \GHCiQException{} -> remoteTHCall (qsPipe s) (EndRecover True) >> h s
      qLookupName isType occ = ghcCmd (LookupName isType occ)
      qReify name = ghcCmd (Reify name)
      qReifyFixity name = ghcCmd (ReifyFixity name)
      qReifyInstances name tys = ghcCmd (ReifyInstances name tys)
      qReifyRoles name = ghcCmd (ReifyRoles name)

      -- To reify annotations, we send GHC the AnnLookup and also the
      -- TypeRep of the thing we're looking for, to avoid needing to
      -- serialize irrelevant annotations.
      qReifyAnnotations :: forall a . Data a => TH.AnnLookup -> GHCiQ [a]
      qReifyAnnotations lookup =
        map (deserializeWithData . B.unpack) <$>
          ghcCmd (ReifyAnnotations lookup typerep)
        where typerep = typeOf (undefined :: a)

      qReifyModule m = ghcCmd (ReifyModule m)
      qReifyConStrictness name = ghcCmd (ReifyConStrictness name)
      qLocation = fromMaybe noLoc . qsLocation <$> getState
      qRunIO m = GHCiQ $ \s -> fmap (,s) m
      qAddDependentFile file = ghcCmd (AddDependentFile file)
      qAddTopDecls decls = ghcCmd (AddTopDecls decls)
      qAddForeignFile str lang = ghcCmd (AddForeignFile str lang)
      qAddModFinalizer fin = GHCiQ (\s -> mkRemoteRef fin >>= return . (, s)) >>=
                             ghcCmd . AddModFinalizer
      qGetQ = GHCiQ $ \s ->
        let lookup :: forall a. Typeable a => Map TypeRep Dynamic -> Maybe a
            lookup m = fromDynamic =<< M.lookup (typeOf (undefined::a)) m
        in return (lookup (qsMap s), s)
      qPutQ k = GHCiQ $ \s ->
        return ((), s { qsMap = M.insert (typeOf k) (toDyn k) (qsMap s) })
      qIsExtEnabled x = ghcCmd (IsExtEnabled x)
      qExtsEnabled = ghcCmd ExtsEnabled

    -- | The implementation of the 'StartTH' message: create
    -- a new IORef QState, and return a RemoteRef to it.
    startTH :: IO (RemoteRef (IORef QState))
    startTH = do
      r <- newIORef (initQState (error "startTH: no pipe"))
      mkRemoteRef r

    -- | Runs the mod finalizers.
    --
    -- The references must be created on the caller process.
    runModFinalizerRefs :: Pipe -> RemoteRef (IORef QState)
                        -> [RemoteRef (TH.Q ())]
                        -> IO ()
    runModFinalizerRefs pipe rstate qrefs = do
      qs <- mapM localRef qrefs
      qstateref <- localRef rstate
      qstate <- readIORef qstateref
      _ <- runGHCiQ (TH.runQ $ sequence_ qs) qstate { qsPipe = pipe }
      return ()

    -- | The implementation of the 'RunTH' message
    runTH
      :: Pipe
      -> RemoteRef (IORef QState)
          -- ^ The TH state, created by 'startTH'
      -> HValueRef
          -- ^ The splice to run
      -> THResultType
          -- ^ What kind of splice it is
      -> Maybe TH.Loc
          -- ^ The source location
      -> IO ByteString
          -- ^ Returns an (encoded) result that depends on the THResultType

    runTH pipe rstate rhv ty mb_loc = do
      hv <- localRef rhv
      case ty of
        THExp -> runTHQ pipe rstate mb_loc (unsafeCoerce hv :: TH.Q TH.Exp)
        THPat -> runTHQ pipe rstate mb_loc (unsafeCoerce hv :: TH.Q TH.Pat)
        THType -> runTHQ pipe rstate mb_loc (unsafeCoerce hv :: TH.Q TH.Type)
        THDec -> runTHQ pipe rstate mb_loc (unsafeCoerce hv :: TH.Q [TH.Dec])
        THAnnWrapper -> do
          hv <- unsafeCoerce <$> localRef rhv
          case hv :: AnnotationWrapper of
            AnnotationWrapper thing -> return $!
              LB.toStrict (runPut (put (toSerialized serializeWithData thing)))

    -- | Run a Q computation.
    runTHQ
      :: Binary a => Pipe -> RemoteRef (IORef QState) -> Maybe TH.Loc -> TH.Q a
      -> IO ByteString
    runTHQ pipe@Pipe{..} rstate mb_loc ghciq = do
      qstateref <- localRef rstate
      qstate <- readIORef qstateref
      let st = qstate { qsLocation = mb_loc, qsPipe = pipe }
      (r,new_state) <- runGHCiQ (TH.runQ ghciq) st
      writeIORef qstateref new_state
      return $! LB.toStrict (runPut (put r))