typecheck/TcForeign.hs

    {-
    (c) The University of Glasgow 2006
    (c) The AQUA Project, Glasgow University, 1998

    \section[TcForeign]{Typechecking \tr{foreign} declarations}

    A foreign declaration is used to either give an externally
    implemented function a Haskell type (and calling interface) or
    give a Haskell function an external calling interface. Either way,
    the range of argument and result types these functions can accommodate
    is restricted to what the outside world understands (read C), and this
    module checks to see if a foreign declaration has got a legal type.
    -}

    {-# LANGUAGE CPP #-}

    module TcForeign
            ( tcForeignImports
            , tcForeignExports

            -- Low-level exports for hooks
            , isForeignImport, isForeignExport
            , tcFImport, tcFExport
            , tcForeignImports'
            , tcCheckFIType, checkCTarget, checkForeignArgs, checkForeignRes
            , normaliseFfiType
            , nonIOok, mustBeIO
            , checkSafe, noCheckSafe
            , tcForeignExports'
            , tcCheckFEType
            ) where

    #include "HsVersions.h"

    import HsSyn

    import TcRnMonad
    import TcHsType
    import TcExpr
    import TcEnv

    import FamInst
    import FamInstEnv
    import Coercion
    import Type
    import ForeignCall
    import ErrUtils
    import Id
    import Name
    import RdrName
    import DataCon
    import TyCon
    import TcType
    import PrelNames
    import DynFlags
    import Outputable
    import Platform
    import SrcLoc
    import Bag
    import Hooks
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Data.Maybe

    -- Defines a binding
    isForeignImport :: LForeignDecl name -> Bool
    isForeignImport (L _ (ForeignImport {})) = True
    isForeignImport _                        = False

    -- Exports a binding
    isForeignExport :: LForeignDecl name -> Bool
    isForeignExport (L _ (ForeignExport {})) = True
    isForeignExport _                        = False

    {-
    Note [Don't recur in normaliseFfiType']
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    normaliseFfiType' is the workhorse for normalising a type used in a foreign
    declaration. If we have

    newtype Age = MkAge Int

    we want to see that Age -> IO () is the same as Int -> IO (). But, we don't
    need to recur on any type parameters, because no paramaterized types (with
    interesting parameters) are marshalable! The full list of marshalable types
    is in the body of boxedMarshalableTyCon in TcType. The only members of that
    list not at kind * are Ptr, FunPtr, and StablePtr, all of which get marshaled
    the same way regardless of type parameter. So, no need to recur into
    parameters.

    Similarly, we don't need to look in AppTy's, because nothing headed by
    an AppTy will be marshalable.

    Note [FFI type roles]
    ~~~~~~~~~~~~~~~~~~~~~
    The 'go' helper function within normaliseFfiType' always produces
    representational coercions. But, in the "children_only" case, we need to
    use these coercions in a TyConAppCo. Accordingly, the roles on the coercions
    must be twiddled to match the expectation of the enclosing TyCon. However,
    we cannot easily go from an R coercion to an N one, so we forbid N roles
    on FFI type constructors. Currently, only two such type constructors exist:
    IO and FunPtr. Thus, this is not an onerous burden.

    If we ever want to lift this restriction, we would need to make 'go' take
    the target role as a parameter. This wouldn't be hard, but it's a complication
    not yet necessary and so is not yet implemented.
    -}

    -- normaliseFfiType takes the type from an FFI declaration, and
    -- evaluates any type synonyms, type functions, and newtypes. However,
    -- we are only allowed to look through newtypes if the constructor is
    -- in scope.  We return a bag of all the newtype constructors thus found.
    -- Always returns a Representational coercion
    normaliseFfiType :: Type -> TcM (Coercion, Type, Bag GlobalRdrElt)
    normaliseFfiType ty
        = do fam_envs <- tcGetFamInstEnvs
             normaliseFfiType' fam_envs ty

    normaliseFfiType' :: FamInstEnvs -> Type -> TcM (Coercion, Type, Bag GlobalRdrElt)
    normaliseFfiType' env ty0 = go initRecTc ty0
      where
        go :: RecTcChecker -> Type -> TcM (Coercion, Type, Bag GlobalRdrElt)
        go rec_nts ty
          | Just ty' <- tcView ty     -- Expand synonyms
          = go rec_nts ty'

          | Just (tc, tys) <- splitTyConApp_maybe ty
          = go_tc_app rec_nts tc tys

          | (bndrs, inner_ty) <- splitForAllTyVarBndrs ty
          , not (null bndrs)
          = do (coi, nty1, gres1) <- go rec_nts inner_ty
               return ( mkHomoForAllCos (binderVars bndrs) coi
                      , mkForAllTys bndrs nty1, gres1 )

          | otherwise -- see Note [Don't recur in normaliseFfiType']
          = return (mkRepReflCo ty, ty, emptyBag)

        go_tc_app :: RecTcChecker -> TyCon -> [Type]
                  -> TcM (Coercion, Type, Bag GlobalRdrElt)
        go_tc_app rec_nts tc tys
            -- We don't want to look through the IO newtype, even if it is
            -- in scope, so we have a special case for it:
            | tc_key `elem` [ioTyConKey, funPtrTyConKey, funTyConKey]
                      -- These *must not* have nominal roles on their parameters!
                      -- See Note [FFI type roles]
            = children_only

            | isNewTyCon tc         -- Expand newtypes
            , Just rec_nts' <- checkRecTc rec_nts tc
                       -- See Note [Expanding newtypes] in TyCon.hs
                       -- We can't just use isRecursiveTyCon; sometimes recursion is ok:
                       --     newtype T = T (Ptr T)
                       --   Here, we don't reject the type for being recursive.
                       -- If this is a recursive newtype then it will normally
                       -- be rejected later as not being a valid FFI type.
            = do { rdr_env <- getGlobalRdrEnv
                 ; case checkNewtypeFFI rdr_env tc of
                     Nothing  -> nothing
                     Just gre -> do { (co', ty', gres) <- go rec_nts' nt_rhs
                                    ; return (mkTransCo nt_co co', ty', gre `consBag` gres) } }

            | isFamilyTyCon tc              -- Expand open tycons
            , (co, ty) <- normaliseTcApp env Representational tc tys
            , not (isReflexiveCo co)
            = do (co', ty', gres) <- go rec_nts ty
                 return (mkTransCo co co', ty', gres)

            | otherwise
            = nothing -- see Note [Don't recur in normaliseFfiType']
            where
              tc_key = getUnique tc
              children_only
                = do xs <- mapM (go rec_nts) tys
                     let (cos, tys', gres) = unzip3 xs
                            -- the (repeat Representational) is because 'go' always
                            -- returns R coercions
                         cos' = zipWith3 downgradeRole (tyConRoles tc)
                                         (repeat Representational) cos
                     return ( mkTyConAppCo Representational tc cos'
                            , mkTyConApp tc tys', unionManyBags gres)
              nt_co  = mkUnbranchedAxInstCo Representational (newTyConCo tc) tys []
              nt_rhs = newTyConInstRhs tc tys

              ty      = mkTyConApp tc tys
              nothing = return (mkRepReflCo ty, ty, emptyBag)

    checkNewtypeFFI :: GlobalRdrEnv -> TyCon -> Maybe GlobalRdrElt
    checkNewtypeFFI rdr_env tc
      | Just con <- tyConSingleDataCon_maybe tc
      , Just gre <- lookupGRE_Name rdr_env (dataConName con)
      = Just gre    -- See Note [Newtype constructor usage in foreign declarations]
      | otherwise
      = Nothing

    {-
    Note [Newtype constructor usage in foreign declarations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    GHC automatically "unwraps" newtype constructors in foreign import/export
    declarations.  In effect that means that a newtype data constructor is
    used even though it is not mentioned expclitly in the source, so we don't
    want to report it as "defined but not used" or "imported but not used".
    eg     newtype D = MkD Int
           foreign import foo :: D -> IO ()
    Here 'MkD' us used.  See Trac #7408.

    GHC also expands type functions during this process, so it's not enough
    just to look at the free variables of the declaration.
    eg     type instance F Bool = D
           foreign import bar :: F Bool -> IO ()
    Here again 'MkD' is used.

    So we really have wait until the type checker to decide what is used.
    That's why tcForeignImports and tecForeignExports return a (Bag GRE)
    for the newtype constructors they see. Then TcRnDriver can add them
    to the module's usages.

    ************************************************************************
    *                                                                      *
    \subsection{Imports}
    *                                                                      *
    ************************************************************************
    -}

    tcForeignImports :: [LForeignDecl Name] -> TcM ([Id], [LForeignDecl Id], Bag GlobalRdrElt)
    tcForeignImports decls
      = getHooked tcForeignImportsHook tcForeignImports' >>= ($ decls)

    tcForeignImports' :: [LForeignDecl Name] -> TcM ([Id], [LForeignDecl Id], Bag GlobalRdrElt)
    -- For the (Bag GlobalRdrElt) result,
    -- see Note [Newtype constructor usage in foreign declarations]
    tcForeignImports' decls
      = do { (ids, decls, gres) <- mapAndUnzip3M tcFImport $
                                   filter isForeignImport decls
           ; return (ids, decls, unionManyBags gres) }

    tcFImport :: LForeignDecl Name -> TcM (Id, LForeignDecl Id, Bag GlobalRdrElt)
    tcFImport (L dloc fo@(ForeignImport { fd_name = L nloc nm, fd_sig_ty = hs_ty
                                        , fd_fi = imp_decl }))
      = setSrcSpan dloc $ addErrCtxt (foreignDeclCtxt fo)  $
        do { sig_ty <- tcHsSigType (ForSigCtxt nm) hs_ty
           ; (norm_co, norm_sig_ty, gres) <- normaliseFfiType sig_ty
           ; let
               -- Drop the foralls before inspecting the
               -- structure of the foreign type.
                 (bndrs, res_ty)   = tcSplitPiTys norm_sig_ty
                 arg_tys           = mapMaybe binderRelevantType_maybe bndrs
                 id                = mkLocalId nm sig_ty
                     -- Use a LocalId to obey the invariant that locally-defined
                     -- things are LocalIds.  However, it does not need zonking,
                     -- (so TcHsSyn.zonkForeignExports ignores it).

           ; imp_decl' <- tcCheckFIType arg_tys res_ty imp_decl
              -- Can't use sig_ty here because sig_ty :: Type and
              -- we need HsType Id hence the undefined
           ; let fi_decl = ForeignImport { fd_name = L nloc id
                                         , fd_sig_ty = undefined
                                         , fd_co = mkSymCo norm_co
                                         , fd_fi = imp_decl' }
           ; return (id, L dloc fi_decl, gres) }
    tcFImport d = pprPanic "tcFImport" (ppr d)

    -- ------------ Checking types for foreign import ----------------------

    tcCheckFIType :: [Type] -> Type -> ForeignImport -> TcM ForeignImport

    tcCheckFIType arg_tys res_ty (CImport (L lc cconv) safety mh l@(CLabel _) src)
      -- Foreign import label
      = do checkCg checkCOrAsmOrLlvmOrInterp
           -- NB check res_ty not sig_ty!
           --    In case sig_ty is (forall a. ForeignPtr a)
           check (isFFILabelTy (mkFunTys arg_tys res_ty)) (illegalForeignTyErr Outputable.empty)
           cconv' <- checkCConv cconv
           return (CImport (L lc cconv') safety mh l src)

    tcCheckFIType arg_tys res_ty (CImport (L lc cconv) safety mh CWrapper src) = do
            -- Foreign wrapper (former f.e.d.)
            -- The type must be of the form ft -> IO (FunPtr ft), where ft is a valid
            -- foreign type.  For legacy reasons ft -> IO (Ptr ft) is accepted, too.
            -- The use of the latter form is DEPRECATED, though.
        checkCg checkCOrAsmOrLlvmOrInterp
        cconv' <- checkCConv cconv
        case arg_tys of
            [arg1_ty] -> do checkForeignArgs isFFIExternalTy arg1_tys
                            checkForeignRes nonIOok  checkSafe isFFIExportResultTy res1_ty
                            checkForeignRes mustBeIO checkSafe (isFFIDynTy arg1_ty) res_ty
                      where
                         (arg1_tys, res1_ty) = tcSplitFunTys arg1_ty
            _ -> addErrTc (illegalForeignTyErr Outputable.empty (text "One argument expected"))
        return (CImport (L lc cconv') safety mh CWrapper src)

    tcCheckFIType arg_tys res_ty idecl@(CImport (L lc cconv) (L ls safety) mh
                                                (CFunction target) src)
      | isDynamicTarget target = do -- Foreign import dynamic
          checkCg checkCOrAsmOrLlvmOrInterp
          cconv' <- checkCConv cconv
          case arg_tys of           -- The first arg must be Ptr or FunPtr
            []                ->
              addErrTc (illegalForeignTyErr Outputable.empty (text "At least one argument expected"))
            (arg1_ty:arg_tys) -> do
              dflags <- getDynFlags
              let curried_res_ty = mkFunTys arg_tys res_ty
              check (isFFIDynTy curried_res_ty arg1_ty)
                    (illegalForeignTyErr argument)
              checkForeignArgs (isFFIArgumentTy dflags safety) arg_tys
              checkForeignRes nonIOok checkSafe (isFFIImportResultTy dflags) res_ty
          return $ CImport (L lc cconv') (L ls safety) mh (CFunction target) src
      | cconv == PrimCallConv = do
          dflags <- getDynFlags
          checkTc (xopt LangExt.GHCForeignImportPrim dflags)
                  (text "Use GHCForeignImportPrim to allow `foreign import prim'.")
          checkCg checkCOrAsmOrLlvmOrInterp
          checkCTarget target
          checkTc (playSafe safety)
                  (text "The safe/unsafe annotation should not be used with `foreign import prim'.")
          checkForeignArgs (isFFIPrimArgumentTy dflags) arg_tys
          -- prim import result is more liberal, allows (#,,#)
          checkForeignRes nonIOok checkSafe (isFFIPrimResultTy dflags) res_ty
          return idecl
      | otherwise = do              -- Normal foreign import
          checkCg checkCOrAsmOrLlvmOrInterp
          cconv' <- checkCConv cconv
          checkCTarget target
          dflags <- getDynFlags
          checkForeignArgs (isFFIArgumentTy dflags safety) arg_tys
          checkForeignRes nonIOok checkSafe (isFFIImportResultTy dflags) res_ty
          checkMissingAmpersand dflags arg_tys res_ty
          case target of
              StaticTarget _ _ _ False
               | not (null arg_tys) ->
                  addErrTc (text "`value' imports cannot have function types")
              _ -> return ()
          return $ CImport (L lc cconv') (L ls safety) mh (CFunction target) src

    -- This makes a convenient place to check
    -- that the C identifier is valid for C
    checkCTarget :: CCallTarget -> TcM ()
    checkCTarget (StaticTarget _ str _ _) = do
        checkCg checkCOrAsmOrLlvmOrInterp
        checkTc (isCLabelString str) (badCName str)

    checkCTarget DynamicTarget = panic "checkCTarget DynamicTarget"

    checkMissingAmpersand :: DynFlags -> [Type] -> Type -> TcM ()
    checkMissingAmpersand dflags arg_tys res_ty
      | null arg_tys && isFunPtrTy res_ty &&
        wopt Opt_WarnDodgyForeignImports dflags
      = addWarn (Reason Opt_WarnDodgyForeignImports)
            (text "possible missing & in foreign import of FunPtr")
      | otherwise
      = return ()

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Exports}
    *                                                                      *
    ************************************************************************
    -}

    tcForeignExports :: [LForeignDecl Name]
                     -> TcM (LHsBinds TcId, [LForeignDecl TcId], Bag GlobalRdrElt)
    tcForeignExports decls =
      getHooked tcForeignExportsHook tcForeignExports' >>= ($ decls)

    tcForeignExports' :: [LForeignDecl Name]
                     -> TcM (LHsBinds TcId, [LForeignDecl TcId], Bag GlobalRdrElt)
    -- For the (Bag GlobalRdrElt) result,
    -- see Note [Newtype constructor usage in foreign declarations]
    tcForeignExports' decls
      = foldlM combine (emptyLHsBinds, [], emptyBag) (filter isForeignExport decls)
      where
       combine (binds, fs, gres1) (L loc fe) = do
           (b, f, gres2) <- setSrcSpan loc (tcFExport fe)
           return (b `consBag` binds, L loc f : fs, gres1 `unionBags` gres2)

    tcFExport :: ForeignDecl Name -> TcM (LHsBind Id, ForeignDecl Id, Bag GlobalRdrElt)
    tcFExport fo@(ForeignExport { fd_name = L loc nm, fd_sig_ty = hs_ty, fd_fe = spec })
      = addErrCtxt (foreignDeclCtxt fo) $ do

        sig_ty <- tcHsSigType (ForSigCtxt nm) hs_ty
        rhs <- tcPolyExpr (nlHsVar nm) sig_ty

        (norm_co, norm_sig_ty, gres) <- normaliseFfiType sig_ty

        spec' <- tcCheckFEType norm_sig_ty spec

               -- we're exporting a function, but at a type possibly more
               -- constrained than its declared/inferred type. Hence the need
               -- to create a local binding which will call the exported function
               -- at a particular type (and, maybe, overloading).

        -- We need to give a name to the new top-level binding that
        -- is *stable* (i.e. the compiler won't change it later),
        -- because this name will be referred to by the C code stub.
        id  <- mkStableIdFromName nm sig_ty loc mkForeignExportOcc
        return ( mkVarBind id rhs
               , ForeignExport { fd_name = L loc id
                               , fd_sig_ty = undefined
                               , fd_co = norm_co, fd_fe = spec' }
               , gres)
    tcFExport d = pprPanic "tcFExport" (ppr d)

    -- ------------ Checking argument types for foreign export ----------------------

    tcCheckFEType :: Type -> ForeignExport -> TcM ForeignExport
    tcCheckFEType sig_ty (CExport (L l (CExportStatic esrc str cconv)) src) = do
        checkCg checkCOrAsmOrLlvm
        checkTc (isCLabelString str) (badCName str)
        cconv' <- checkCConv cconv
        checkForeignArgs isFFIExternalTy arg_tys
        checkForeignRes nonIOok noCheckSafe isFFIExportResultTy res_ty
        return (CExport (L l (CExportStatic esrc str cconv')) src)
      where
          -- Drop the foralls before inspecting n
          -- the structure of the foreign type.
        (bndrs, res_ty) = tcSplitPiTys sig_ty
        arg_tys         = mapMaybe binderRelevantType_maybe bndrs

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Miscellaneous}
    *                                                                      *
    ************************************************************************
    -}

    ------------ Checking argument types for foreign import ----------------------
    checkForeignArgs :: (Type -> Validity) -> [Type] -> TcM ()
    checkForeignArgs pred tys = mapM_ go tys
      where
        go ty = check (pred ty) (illegalForeignTyErr argument)

    ------------ Checking result types for foreign calls ----------------------
    -- | Check that the type has the form
    --    (IO t) or (t) , and that t satisfies the given predicate.
    -- When calling this function, any newtype wrappers (should) have been
    -- already dealt with by normaliseFfiType.
    --
    -- We also check that the Safe Haskell condition of FFI imports having
    -- results in the IO monad holds.
    --
    checkForeignRes :: Bool -> Bool -> (Type -> Validity) -> Type -> TcM ()
    checkForeignRes non_io_result_ok check_safe pred_res_ty ty
      | Just (_, res_ty) <- tcSplitIOType_maybe ty
      =     -- Got an IO result type, that's always fine!
         check (pred_res_ty res_ty) (illegalForeignTyErr result)

      -- Case for non-IO result type with FFI Import
      | not non_io_result_ok
      = addErrTc $ illegalForeignTyErr result (text "IO result type expected")

      | otherwise
      = do { dflags <- getDynFlags
           ; case pred_res_ty ty of
                    -- Handle normal typecheck fail, we want to handle this first and
                    -- only report safe haskell errors if the normal type check is OK.
               NotValid msg -> addErrTc $ illegalForeignTyErr result msg

               -- handle safe infer fail
               _ | check_safe && safeInferOn dflags
                   -> recordUnsafeInfer emptyBag

               -- handle safe language typecheck fail
               _ | check_safe && safeLanguageOn dflags
                   -> addErrTc (illegalForeignTyErr result safeHsErr)

               -- success! non-IO return is fine
               _ -> return () }
      where
        safeHsErr =
          text "Safe Haskell is on, all FFI imports must be in the IO monad"

    nonIOok, mustBeIO :: Bool
    nonIOok  = True
    mustBeIO = False

    checkSafe, noCheckSafe :: Bool
    checkSafe   = True
    noCheckSafe = False

    -- Checking a supported backend is in use

    checkCOrAsmOrLlvm :: HscTarget -> Validity
    checkCOrAsmOrLlvm HscC    = IsValid
    checkCOrAsmOrLlvm HscAsm  = IsValid
    checkCOrAsmOrLlvm HscLlvm = IsValid
    checkCOrAsmOrLlvm _
      = NotValid (text "requires unregisterised, llvm (-fllvm) or native code generation (-fasm)")

    checkCOrAsmOrLlvmOrInterp :: HscTarget -> Validity
    checkCOrAsmOrLlvmOrInterp HscC           = IsValid
    checkCOrAsmOrLlvmOrInterp HscAsm         = IsValid
    checkCOrAsmOrLlvmOrInterp HscLlvm        = IsValid
    checkCOrAsmOrLlvmOrInterp HscInterpreted = IsValid
    checkCOrAsmOrLlvmOrInterp _
      = NotValid (text "requires interpreted, unregisterised, llvm or native code generation")

    checkCg :: (HscTarget -> Validity) -> TcM ()
    checkCg check = do
        dflags <- getDynFlags
        let target = hscTarget dflags
        case target of
          HscNothing -> return ()
          _ ->
            case check target of
              IsValid      -> return ()
              NotValid err -> addErrTc (text "Illegal foreign declaration:" <+> err)

    -- Calling conventions

    checkCConv :: CCallConv -> TcM CCallConv
    checkCConv CCallConv    = return CCallConv
    checkCConv CApiConv     = return CApiConv
    checkCConv StdCallConv  = do dflags <- getDynFlags
                                 let platform = targetPlatform dflags
                                 if platformArch platform == ArchX86
                                     then return StdCallConv
                                     else do -- This is a warning, not an error. see #3336
                                             when (wopt Opt_WarnUnsupportedCallingConventions dflags) $
                                                 addWarnTc (Reason Opt_WarnUnsupportedCallingConventions)
                                                     (text "the 'stdcall' calling convention is unsupported on this platform," $$ text "treating as ccall")
                                             return CCallConv
    checkCConv PrimCallConv = do addErrTc (text "The `prim' calling convention can only be used with `foreign import'")
                                 return PrimCallConv
    checkCConv JavaScriptCallConv = do dflags <- getDynFlags
                                       if platformArch (targetPlatform dflags) == ArchJavaScript
                                           then return JavaScriptCallConv
                                           else do addErrTc (text "The `javascript' calling convention is unsupported on this platform")
                                                   return JavaScriptCallConv

    -- Warnings

    check :: Validity -> (MsgDoc -> MsgDoc) -> TcM ()
    check IsValid _             = return ()
    check (NotValid doc) err_fn = addErrTc (err_fn doc)

    illegalForeignTyErr :: SDoc -> SDoc -> SDoc
    illegalForeignTyErr arg_or_res extra
      = hang msg 2 extra
      where
        msg = hsep [ text "Unacceptable", arg_or_res
                   , text "type in foreign declaration:"]

    -- Used for 'arg_or_res' argument to illegalForeignTyErr
    argument, result :: SDoc
    argument = text "argument"
    result   = text "result"

    badCName :: CLabelString -> MsgDoc
    badCName target
      = sep [quotes (ppr target) <+> text "is not a valid C identifier"]

    foreignDeclCtxt :: ForeignDecl Name -> SDoc
    foreignDeclCtxt fo
      = hang (text "When checking declaration:")
           2 (ppr fo)