GHCi/Message.hs

    {-# LANGUAGE GADTs, DeriveGeneric, StandaloneDeriving, ScopedTypeVariables,
        GeneralizedNewtypeDeriving, ExistentialQuantification, RecordWildCards #-}
    {-# OPTIONS_GHC -fno-warn-name-shadowing -fno-warn-orphans #-}

    -- |
    -- Remote GHCi message types and serialization.
    --
    -- For details on Remote GHCi, see Note [Remote GHCi] in
    -- compiler/GHC/Runtime/Interpreter.hs.
    --
    module GHCi.Message
      ( Message(..), Msg(..)
      , THMessage(..), THMsg(..)
      , QResult(..)
      , EvalStatus_(..), EvalStatus, EvalResult(..), EvalOpts(..), EvalExpr(..)
      , SerializableException(..)
      , toSerializableException, fromSerializableException
      , THResult(..), THResultType(..)
      , ResumeContext(..)
      , QState(..)
      , getMessage, putMessage, getTHMessage, putTHMessage
      , Pipe(..), remoteCall, remoteTHCall, readPipe, writePipe
      ) where

    import Prelude -- See note [Why do we import Prelude here?]
    import GHCi.RemoteTypes
    import GHCi.FFI
    import GHCi.TH.Binary () -- For Binary instances
    import GHCi.BreakArray

    import GHC.LanguageExtensions
    import GHC.Exts.Heap
    import GHC.ForeignSrcLang
    import GHC.Fingerprint
    import Control.Concurrent
    import Control.Exception
    import Data.Binary
    import Data.Binary.Get
    import Data.Binary.Put
    import Data.ByteString (ByteString)
    import qualified Data.ByteString as B
    import qualified Data.ByteString.Lazy as LB
    import Data.Dynamic
    import Data.Typeable (TypeRep)
    import Data.IORef
    import Data.Map (Map)
    import Foreign
    import GHC.Generics
    import GHC.Stack.CCS
    import qualified Language.Haskell.TH        as TH
    import qualified Language.Haskell.TH.Syntax as TH
    import System.Exit
    import System.IO
    import System.IO.Error

    -- -----------------------------------------------------------------------------
    -- The RPC protocol between GHC and the interactive server

    -- | A @Message a@ is a message that returns a value of type @a@.
    -- These are requests sent from GHC to the server.
    data Message a where
      -- | Exit the iserv process
      Shutdown :: Message ()
      RtsRevertCAFs :: Message ()

      -- RTS Linker -------------------------------------------

      -- These all invoke the corresponding functions in the RTS Linker API.
      InitLinker :: Message ()
      LookupSymbol :: String -> Message (Maybe (RemotePtr ()))
      LookupClosure :: String -> Message (Maybe HValueRef)
      LoadDLL :: String -> Message (Maybe String)
      LoadArchive :: String -> Message () -- error?
      LoadObj :: String -> Message () -- error?
      UnloadObj :: String -> Message () -- error?
      AddLibrarySearchPath :: String -> Message (RemotePtr ())
      RemoveLibrarySearchPath :: RemotePtr () -> Message Bool
      ResolveObjs :: Message Bool
      FindSystemLibrary :: String -> Message (Maybe String)

      -- Interpreter -------------------------------------------

      -- | Create a set of BCO objects, and return HValueRefs to them
      -- Note: Each ByteString contains a Binary-encoded [ResolvedBCO], not
      -- a ResolvedBCO. The list is to allow us to serialise the ResolvedBCOs
      -- in parallel. See @createBCOs@ in compiler/GHC/Runtime/Interpreter.hs.
      CreateBCOs :: [LB.ByteString] -> Message [HValueRef]

      -- | Release 'HValueRef's
      FreeHValueRefs :: [HValueRef] -> Message ()

      -- | Add entries to the Static Pointer Table
      AddSptEntry :: Fingerprint -> HValueRef -> Message ()

      -- | Malloc some data and return a 'RemotePtr' to it
      MallocData :: ByteString -> Message (RemotePtr ())
      MallocStrings :: [ByteString] -> Message [RemotePtr ()]

      -- | Calls 'GHCi.FFI.prepareForeignCall'
      PrepFFI :: FFIConv -> [FFIType] -> FFIType -> Message (RemotePtr C_ffi_cif)

      -- | Free data previously created by 'PrepFFI'
      FreeFFI :: RemotePtr C_ffi_cif -> Message ()

      -- | Create an info table for a constructor
      MkConInfoTable
       :: Bool    -- TABLES_NEXT_TO_CODE
       -> Int     -- ptr words
       -> Int     -- non-ptr words
       -> Int     -- constr tag
       -> Int     -- pointer tag
       -> ByteString -- constructor desccription
       -> Message (RemotePtr StgInfoTable)

      -- | Evaluate a statement
      EvalStmt
        :: EvalOpts
        -> EvalExpr HValueRef {- IO [a] -}
        -> Message (EvalStatus [HValueRef]) {- [a] -}

      -- | Resume evaluation of a statement after a breakpoint
      ResumeStmt
       :: EvalOpts
       -> RemoteRef (ResumeContext [HValueRef])
       -> Message (EvalStatus [HValueRef])

      -- | Abandon evaluation of a statement after a breakpoint
      AbandonStmt
       :: RemoteRef (ResumeContext [HValueRef])
       -> Message ()

      -- | Evaluate something of type @IO String@
      EvalString
        :: HValueRef {- IO String -}
        -> Message (EvalResult String)

      -- | Evaluate something of type @String -> IO String@
      EvalStringToString
        :: HValueRef {- String -> IO String -}
        -> String
        -> Message (EvalResult String)

      -- | Evaluate something of type @IO ()@
      EvalIO
       :: HValueRef {- IO a -}
       -> Message (EvalResult ())

      -- | Create a set of CostCentres with the same module name
      MkCostCentres
       :: String     -- module, RemotePtr so it can be shared
       -> [(String,String)] -- (name, SrcSpan)
       -> Message [RemotePtr CostCentre]

      -- | Show a 'CostCentreStack' as a @[String]@
      CostCentreStackInfo
       :: RemotePtr CostCentreStack
       -> Message [String]

      -- | Create a new array of breakpoint flags
      NewBreakArray
       :: Int                               -- size
       -> Message (RemoteRef BreakArray)

      -- | Enable a breakpoint
      EnableBreakpoint
       :: RemoteRef BreakArray
       -> Int                               -- index
       -> Bool                              -- on or off
       -> Message ()

      -- | Query the status of a breakpoint (True <=> enabled)
      BreakpointStatus
       :: RemoteRef BreakArray
       -> Int                               -- index
       -> Message Bool                      -- True <=> enabled

      -- | Get a reference to a free variable at a breakpoint
      GetBreakpointVar
       :: HValueRef                         -- the AP_STACK from EvalBreak
       -> Int
       -> Message (Maybe HValueRef)

      -- Template Haskell -------------------------------------------
      -- For more details on how TH works with Remote GHCi, see
      -- Note [Remote Template Haskell] in libraries/ghci/GHCi/TH.hs.

      -- | Start a new TH module, return a state token that should be
      StartTH :: Message (RemoteRef (IORef QState))

      -- | Evaluate a TH computation.
      --
      -- Returns a ByteString, because we have to force the result
      -- before returning it to ensure there are no errors lurking
      -- in it.  The TH types don't have NFData instances, and even if
      -- they did, we have to serialize the value anyway, so we might
      -- as well serialize it to force it.
      RunTH
       :: RemoteRef (IORef QState)
       -> HValueRef {- e.g. TH.Q TH.Exp -}
       -> THResultType
       -> Maybe TH.Loc
       -> Message (QResult ByteString)

      -- | Run the given mod finalizers.
      RunModFinalizers :: RemoteRef (IORef QState)
                       -> [RemoteRef (TH.Q ())]
                       -> Message (QResult ())

      -- | Remote interface to GHC.Exts.Heap.getClosureData. This is used by
      -- the GHCi debugger to inspect values in the heap for :print and
      -- type reconstruction.
      GetClosure
        :: HValueRef
        -> Message (GenClosure HValueRef)

      -- | Evaluate something. This is used to support :force in GHCi.
      Seq
        :: HValueRef
        -> Message (EvalStatus ())

      -- | Resume forcing a free variable in a breakpoint (#2950)
      ResumeSeq
        :: RemoteRef (ResumeContext ())
        -> Message (EvalStatus ())

    deriving instance Show (Message a)

    -- | Template Haskell return values
    data QResult a
      = QDone a
        -- ^ RunTH finished successfully; return value follows
      | QException String
        -- ^ RunTH threw an exception
      | QFail String
        -- ^ RunTH called 'fail'
      deriving (Generic, Show)

    instance Binary a => Binary (QResult a)

    -- | Messages sent back to GHC from GHCi.TH, to implement the methods
    -- of 'Quasi'.  For an overview of how TH works with Remote GHCi, see
    -- Note [Remote Template Haskell] in GHCi.TH.
    data THMessage a where
      NewName :: String -> THMessage (THResult TH.Name)
      Report :: Bool -> String -> THMessage (THResult ())
      LookupName :: Bool -> String -> THMessage (THResult (Maybe TH.Name))
      Reify :: TH.Name -> THMessage (THResult TH.Info)
      ReifyFixity :: TH.Name -> THMessage (THResult (Maybe TH.Fixity))
      ReifyType :: TH.Name -> THMessage (THResult TH.Type)
      ReifyInstances :: TH.Name -> [TH.Type] -> THMessage (THResult [TH.Dec])
      ReifyRoles :: TH.Name -> THMessage (THResult [TH.Role])
      ReifyAnnotations :: TH.AnnLookup -> TypeRep
        -> THMessage (THResult [ByteString])
      ReifyModule :: TH.Module -> THMessage (THResult TH.ModuleInfo)
      ReifyConStrictness :: TH.Name -> THMessage (THResult [TH.DecidedStrictness])

      AddDependentFile :: FilePath -> THMessage (THResult ())
      AddTempFile :: String -> THMessage (THResult FilePath)
      AddModFinalizer :: RemoteRef (TH.Q ()) -> THMessage (THResult ())
      AddCorePlugin :: String -> THMessage (THResult ())
      AddTopDecls :: [TH.Dec] -> THMessage (THResult ())
      AddForeignFilePath :: ForeignSrcLang -> FilePath -> THMessage (THResult ())
      IsExtEnabled :: Extension -> THMessage (THResult Bool)
      ExtsEnabled :: THMessage (THResult [Extension])

      StartRecover :: THMessage ()
      EndRecover :: Bool -> THMessage ()
      FailIfErrs :: THMessage (THResult ())

      -- | Indicates that this RunTH is finished, and the next message
      -- will be the result of RunTH (a QResult).
      RunTHDone :: THMessage ()

    deriving instance Show (THMessage a)

    data THMsg = forall a . (Binary a, Show a) => THMsg (THMessage a)

    getTHMessage :: Get THMsg
    getTHMessage = do
      b <- getWord8
      case b of
        0  -> THMsg <$> NewName <$> get
        1  -> THMsg <$> (Report <$> get <*> get)
        2  -> THMsg <$> (LookupName <$> get <*> get)
        3  -> THMsg <$> Reify <$> get
        4  -> THMsg <$> ReifyFixity <$> get
        5  -> THMsg <$> (ReifyInstances <$> get <*> get)
        6  -> THMsg <$> ReifyRoles <$> get
        7  -> THMsg <$> (ReifyAnnotations <$> get <*> get)
        8  -> THMsg <$> ReifyModule <$> get
        9  -> THMsg <$> ReifyConStrictness <$> get
        10 -> THMsg <$> AddDependentFile <$> get
        11 -> THMsg <$> AddTempFile <$> get
        12 -> THMsg <$> AddTopDecls <$> get
        13 -> THMsg <$> (IsExtEnabled <$> get)
        14 -> THMsg <$> return ExtsEnabled
        15 -> THMsg <$> return StartRecover
        16 -> THMsg <$> EndRecover <$> get
        17 -> THMsg <$> return FailIfErrs
        18 -> return (THMsg RunTHDone)
        19 -> THMsg <$> AddModFinalizer <$> get
        20 -> THMsg <$> (AddForeignFilePath <$> get <*> get)
        21 -> THMsg <$> AddCorePlugin <$> get
        22 -> THMsg <$> ReifyType <$> get
        n -> error ("getTHMessage: unknown message " ++ show n)

    putTHMessage :: THMessage a -> Put
    putTHMessage m = case m of
      NewName a                   -> putWord8 0  >> put a
      Report a b                  -> putWord8 1  >> put a >> put b
      LookupName a b              -> putWord8 2  >> put a >> put b
      Reify a                     -> putWord8 3  >> put a
      ReifyFixity a               -> putWord8 4  >> put a
      ReifyInstances a b          -> putWord8 5  >> put a >> put b
      ReifyRoles a                -> putWord8 6  >> put a
      ReifyAnnotations a b        -> putWord8 7  >> put a >> put b
      ReifyModule a               -> putWord8 8  >> put a
      ReifyConStrictness a        -> putWord8 9  >> put a
      AddDependentFile a          -> putWord8 10 >> put a
      AddTempFile a               -> putWord8 11 >> put a
      AddTopDecls a               -> putWord8 12 >> put a
      IsExtEnabled a              -> putWord8 13 >> put a
      ExtsEnabled                 -> putWord8 14
      StartRecover                -> putWord8 15
      EndRecover a                -> putWord8 16 >> put a
      FailIfErrs                  -> putWord8 17
      RunTHDone                   -> putWord8 18
      AddModFinalizer a           -> putWord8 19 >> put a
      AddForeignFilePath lang a   -> putWord8 20 >> put lang >> put a
      AddCorePlugin a             -> putWord8 21 >> put a
      ReifyType a                 -> putWord8 22 >> put a

    data EvalOpts = EvalOpts
      { useSandboxThread :: Bool
      , singleStep :: Bool
      , breakOnException :: Bool
      , breakOnError :: Bool
      }
      deriving (Generic, Show)

    instance Binary EvalOpts

    data ResumeContext a = ResumeContext
      { resumeBreakMVar :: MVar ()
      , resumeStatusMVar :: MVar (EvalStatus a)
      , resumeThreadId :: ThreadId
      }

    -- | We can pass simple expressions to EvalStmt, consisting of values
    -- and application.  This allows us to wrap the statement to be
    -- executed in another function, which is used by GHCi to implement
    -- :set args and :set prog.  It might be worthwhile to extend this
    -- little language in the future.
    data EvalExpr a
      = EvalThis a
      | EvalApp (EvalExpr a) (EvalExpr a)
      deriving (Generic, Show)

    instance Binary a => Binary (EvalExpr a)

    type EvalStatus a = EvalStatus_ a a

    data EvalStatus_ a b
      = EvalComplete Word64 (EvalResult a)
      | EvalBreak Bool
           HValueRef{- AP_STACK -}
           Int {- break index -}
           Int {- uniq of ModuleName -}
           (RemoteRef (ResumeContext b))
           (RemotePtr CostCentreStack) -- Cost centre stack
      deriving (Generic, Show)

    instance Binary a => Binary (EvalStatus_ a b)

    data EvalResult a
      = EvalException SerializableException
      | EvalSuccess a
      deriving (Generic, Show)

    instance Binary a => Binary (EvalResult a)

    -- SomeException can't be serialized because it contains dynamic
    -- types.  However, we do very limited things with the exceptions that
    -- are thrown by interpreted computations:
    --
    -- * We print them, e.g. "*** Exception: <something>"
    -- * UserInterrupt has a special meaning
    -- * In ghc -e, exitWith should exit with the appropriate exit code
    --
    -- So all we need to do is distinguish UserInterrupt and ExitCode, and
    -- all other exceptions can be represented by their 'show' string.
    --
    data SerializableException
      = EUserInterrupt
      | EExitCode ExitCode
      | EOtherException String
      deriving (Generic, Show)

    toSerializableException :: SomeException -> SerializableException
    toSerializableException ex
      | Just UserInterrupt <- fromException ex  = EUserInterrupt
      | Just (ec::ExitCode) <- fromException ex = (EExitCode ec)
      | otherwise = EOtherException (show (ex :: SomeException))

    fromSerializableException :: SerializableException -> SomeException
    fromSerializableException EUserInterrupt = toException UserInterrupt
    fromSerializableException (EExitCode c) = toException c
    fromSerializableException (EOtherException str) = toException (ErrorCall str)

    instance Binary ExitCode
    instance Binary SerializableException

    data THResult a
      = THException String
      | THComplete a
      deriving (Generic, Show)

    instance Binary a => Binary (THResult a)

    data THResultType = THExp | THPat | THType | THDec | THAnnWrapper
      deriving (Enum, Show, Generic)

    instance Binary THResultType

    -- | The server-side Template Haskell state.  This is created by the
    -- StartTH message.  A new one is created per module that GHC
    -- typechecks.
    data QState = QState
      { qsMap        :: Map TypeRep Dynamic
           -- ^ persistent data between splices in a module
      , qsLocation   :: Maybe TH.Loc
           -- ^ location for current splice, if any
      , qsPipe :: Pipe
           -- ^ pipe to communicate with GHC
      }
    instance Show QState where show _ = "<QState>"

    -- Orphan instances of Binary for Ptr / FunPtr by conversion to Word64.
    -- This is to support Binary StgInfoTable which includes these.
    instance Binary (Ptr a) where
      put p = put (fromIntegral (ptrToWordPtr p) :: Word64)
      get = (wordPtrToPtr . fromIntegral) <$> (get :: Get Word64)

    instance Binary (FunPtr a) where
      put = put . castFunPtrToPtr
      get = castPtrToFunPtr <$> get

    -- Binary instances to support the GetClosure message
    instance Binary StgInfoTable
    instance Binary ClosureType
    instance Binary PrimType
    instance Binary a => Binary (GenClosure a)

    data Msg = forall a . (Binary a, Show a) => Msg (Message a)

    getMessage :: Get Msg
    getMessage = do
        b <- getWord8
        case b of
          0  -> Msg <$> return Shutdown
          1  -> Msg <$> return InitLinker
          2  -> Msg <$> LookupSymbol <$> get
          3  -> Msg <$> LookupClosure <$> get
          4  -> Msg <$> LoadDLL <$> get
          5  -> Msg <$> LoadArchive <$> get
          6  -> Msg <$> LoadObj <$> get
          7  -> Msg <$> UnloadObj <$> get
          8  -> Msg <$> AddLibrarySearchPath <$> get
          9  -> Msg <$> RemoveLibrarySearchPath <$> get
          10 -> Msg <$> return ResolveObjs
          11 -> Msg <$> FindSystemLibrary <$> get
          12 -> Msg <$> CreateBCOs <$> get
          13 -> Msg <$> FreeHValueRefs <$> get
          14 -> Msg <$> MallocData <$> get
          15 -> Msg <$> MallocStrings <$> get
          16 -> Msg <$> (PrepFFI <$> get <*> get <*> get)
          17 -> Msg <$> FreeFFI <$> get
          18 -> Msg <$> (MkConInfoTable <$> get <*> get <*> get <*> get <*> get <*> get)
          19 -> Msg <$> (EvalStmt <$> get <*> get)
          20 -> Msg <$> (ResumeStmt <$> get <*> get)
          21 -> Msg <$> (AbandonStmt <$> get)
          22 -> Msg <$> (EvalString <$> get)
          23 -> Msg <$> (EvalStringToString <$> get <*> get)
          24 -> Msg <$> (EvalIO <$> get)
          25 -> Msg <$> (MkCostCentres <$> get <*> get)
          26 -> Msg <$> (CostCentreStackInfo <$> get)
          27 -> Msg <$> (NewBreakArray <$> get)
          28 -> Msg <$> (EnableBreakpoint <$> get <*> get <*> get)
          29 -> Msg <$> (BreakpointStatus <$> get <*> get)
          30 -> Msg <$> (GetBreakpointVar <$> get <*> get)
          31 -> Msg <$> return StartTH
          32 -> Msg <$> (RunModFinalizers <$> get <*> get)
          33 -> Msg <$> (AddSptEntry <$> get <*> get)
          34 -> Msg <$> (RunTH <$> get <*> get <*> get <*> get)
          35 -> Msg <$> (GetClosure <$> get)
          36 -> Msg <$> (Seq <$> get)
          37 -> Msg <$> return RtsRevertCAFs
          38 -> Msg <$> (ResumeSeq <$> get)
          _  -> error $ "Unknown Message code " ++ (show b)

    putMessage :: Message a -> Put
    putMessage m = case m of
      Shutdown                    -> putWord8 0
      InitLinker                  -> putWord8 1
      LookupSymbol str            -> putWord8 2  >> put str
      LookupClosure str           -> putWord8 3  >> put str
      LoadDLL str                 -> putWord8 4  >> put str
      LoadArchive str             -> putWord8 5  >> put str
      LoadObj str                 -> putWord8 6  >> put str
      UnloadObj str               -> putWord8 7  >> put str
      AddLibrarySearchPath str    -> putWord8 8  >> put str
      RemoveLibrarySearchPath ptr -> putWord8 9  >> put ptr
      ResolveObjs                 -> putWord8 10
      FindSystemLibrary str       -> putWord8 11 >> put str
      CreateBCOs bco              -> putWord8 12 >> put bco
      FreeHValueRefs val          -> putWord8 13 >> put val
      MallocData bs               -> putWord8 14 >> put bs
      MallocStrings bss           -> putWord8 15 >> put bss
      PrepFFI conv args res       -> putWord8 16 >> put conv >> put args >> put res
      FreeFFI p                   -> putWord8 17 >> put p
      MkConInfoTable tc p n t pt d -> putWord8 18 >> put tc >> put p >> put n >> put t >> put pt >> put d
      EvalStmt opts val           -> putWord8 19 >> put opts >> put val
      ResumeStmt opts val         -> putWord8 20 >> put opts >> put val
      AbandonStmt val             -> putWord8 21 >> put val
      EvalString val              -> putWord8 22 >> put val
      EvalStringToString str val  -> putWord8 23 >> put str >> put val
      EvalIO val                  -> putWord8 24 >> put val
      MkCostCentres mod ccs       -> putWord8 25 >> put mod >> put ccs
      CostCentreStackInfo ptr     -> putWord8 26 >> put ptr
      NewBreakArray sz            -> putWord8 27 >> put sz
      EnableBreakpoint arr ix b   -> putWord8 28 >> put arr >> put ix >> put b
      BreakpointStatus arr ix     -> putWord8 29 >> put arr >> put ix
      GetBreakpointVar a b        -> putWord8 30 >> put a >> put b
      StartTH                     -> putWord8 31
      RunModFinalizers a b        -> putWord8 32 >> put a >> put b
      AddSptEntry a b             -> putWord8 33 >> put a >> put b
      RunTH st q loc ty           -> putWord8 34 >> put st >> put q >> put loc >> put ty
      GetClosure a                -> putWord8 35 >> put a
      Seq a                       -> putWord8 36 >> put a
      RtsRevertCAFs               -> putWord8 37
      ResumeSeq a                 -> putWord8 38 >> put a

    -- -----------------------------------------------------------------------------
    -- Reading/writing messages

    data Pipe = Pipe
      { pipeRead :: Handle
      , pipeWrite ::  Handle
      , pipeLeftovers :: IORef (Maybe ByteString)
      }

    remoteCall :: Binary a => Pipe -> Message a -> IO a
    remoteCall pipe msg = do
      writePipe pipe (putMessage msg)
      readPipe pipe get

    remoteTHCall :: Binary a => Pipe -> THMessage a -> IO a
    remoteTHCall pipe msg = do
      writePipe pipe (putTHMessage msg)
      readPipe pipe get

    writePipe :: Pipe -> Put -> IO ()
    writePipe Pipe{..} put
      | LB.null bs = return ()
      | otherwise  = do
        LB.hPut pipeWrite bs
        hFlush pipeWrite
     where
      bs = runPut put

    readPipe :: Pipe -> Get a -> IO a
    readPipe Pipe{..} get = do
      leftovers <- readIORef pipeLeftovers
      m <- getBin pipeRead get leftovers
      case m of
        Nothing -> throw $
          mkIOError eofErrorType "GHCi.Message.remoteCall" (Just pipeRead) Nothing
        Just (result, new_leftovers) -> do
          writeIORef pipeLeftovers new_leftovers
          return result

    getBin
      :: Handle -> Get a -> Maybe ByteString
      -> IO (Maybe (a, Maybe ByteString))

    getBin h get leftover = go leftover (runGetIncremental get)
     where
       go Nothing (Done leftover _ msg) =
         return (Just (msg, if B.null leftover then Nothing else Just leftover))
       go _ Done{} = throwIO (ErrorCall "getBin: Done with leftovers")
       go (Just leftover) (Partial fun) = do
         go Nothing (fun (Just leftover))
       go Nothing (Partial fun) = do
         -- putStrLn "before hGetSome"
         b <- B.hGetSome h (32*1024)
         -- printf "hGetSome: %d\n" (B.length b)
         if B.null b
            then return Nothing
            else go Nothing (fun (Just b))
       go _lft (Fail _rest _off str) =
         throwIO (ErrorCall ("getBin: " ++ str))