path: root/compiler/GHC/Runtime/Eval.hs

{-# LANGUAGE CPP, MagicHash, RecordWildCards, BangPatterns #-}
{-# LANGUAGE LambdaCase #-}

{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

-- -----------------------------------------------------------------------------
--
-- (c) The University of Glasgow, 2005-2007
--
-- Running statements interactively
--
-- -----------------------------------------------------------------------------

module GHC.Runtime.Eval (
        Resume(..), History(..),
        execStmt, execStmt', ExecOptions(..), execOptions, ExecResult(..), resumeExec,
        runDecls, runDeclsWithLocation, runParsedDecls,
        isStmt, hasImport, isImport, isDecl,
        parseImportDecl, SingleStep(..),
        abandon, abandonAll,
        getResumeContext,
        getHistorySpan,
        getModBreaks,
        getHistoryModule,
        back, forward,
        setContext, getContext,
        availsToGlobalRdrEnv,
        getNamesInScope,
        getRdrNamesInScope,
        moduleIsInterpreted,
        getInfo,
        exprType,
        typeKind,
        parseName,
        parseInstanceHead,
        getInstancesForType,
        getDocs,
        GetDocsFailure(..),
        showModule,
        moduleIsBootOrNotObjectLinkable,
        parseExpr, compileParsedExpr,
        compileExpr, dynCompileExpr,
        compileExprRemote, compileParsedExprRemote,
        Term(..), obtainTermFromId, obtainTermFromVal, reconstructType
        ) where

#include "HsVersions.h"

import GHC.Prelude

import GHC.Runtime.Eval.Types

import GHC.Runtime.Interpreter as GHCi
import GHC.Runtime.Interpreter.Types
import GHCi.Message
import GHCi.RemoteTypes
import GHC.Driver.Monad
import GHC.Driver.Main
import GHC.Hs
import GHC.Driver.Types
import GHC.Core.InstEnv
import GHC.Iface.Env       ( newInteractiveBinder )
import GHC.Core.FamInstEnv ( FamInst )
import GHC.Core.FVs        ( orphNamesOfFamInst )
import GHC.Core.TyCon
import GHC.Core.Type       hiding( typeKind )
import qualified GHC.Core.Type as Type
import GHC.Types.RepType
import GHC.Tc.Utils.TcType
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin
import GHC.Core.Predicate
import GHC.Types.Var
import GHC.Types.Id as Id
import GHC.Types.Name      hiding ( varName )
import GHC.Types.Name.Set
import GHC.Types.Avail
import GHC.Types.Name.Reader
import GHC.Types.Var.Env
import GHC.ByteCode.Types
import GHC.Runtime.Linker as Linker
import GHC.Driver.Session
import GHC.Driver.Ppr
import GHC.LanguageExtensions
import GHC.Types.Unique
import GHC.Types.Unique.Supply
import GHC.Utils.Monad
import GHC.Unit.Module
import GHC.Builtin.Names ( toDynName, pretendNameIsInScope )
import GHC.Builtin.Types ( isCTupleTyConName )
import GHC.Utils.Panic
import GHC.Data.Maybe
import GHC.Utils.Error
import GHC.Types.SrcLoc
import GHC.Runtime.Heap.Inspect
import GHC.Utils.Outputable
import GHC.Data.FastString
import GHC.Data.Bag
import GHC.Utils.Misc
import qualified GHC.Parser.Lexer as Lexer (P (..), ParseResult(..), unP, mkPStatePure)
import GHC.Parser.Lexer (ParserFlags)
import qualified GHC.Parser       as Parser (parseStmt, parseModule, parseDeclaration, parseImport)

import System.Directory
import Data.Dynamic
import Data.Either
import qualified Data.IntMap as IntMap
import Data.List (find,intercalate)
import Data.Map (Map)
import qualified Data.Map as Map
import GHC.Data.StringBuffer (stringToStringBuffer)
import Control.Monad
import Control.Monad.Catch as MC
import Data.Array
import GHC.Utils.Exception
import Unsafe.Coerce ( unsafeCoerce )

import GHC.Tc.Module ( runTcInteractive, tcRnType, loadUnqualIfaces )
import GHC.Tc.Utils.Zonk ( ZonkFlexi (SkolemiseFlexi) )
import GHC.Tc.Utils.Env (tcGetInstEnvs)
import GHC.Tc.Utils.Instantiate (instDFunType)
import GHC.Tc.Solver (simplifyWantedsTcM)
import GHC.Tc.Utils.Monad
import GHC.Core.Class (classTyCon)

-- -----------------------------------------------------------------------------
-- running a statement interactively

getResumeContext :: GhcMonad m => m [Resume]
getResumeContext = withSession (return . ic_resume . hsc_IC)

mkHistory :: HscEnv -> ForeignHValue -> BreakInfo -> History
mkHistory hsc_env hval bi = History hval bi (findEnclosingDecls hsc_env bi)

getHistoryModule :: History -> Module
getHistoryModule = breakInfo_module . historyBreakInfo

getHistorySpan :: HscEnv -> History -> SrcSpan
getHistorySpan hsc_env History{..} =
  let BreakInfo{..} = historyBreakInfo in
  case lookupHpt (hsc_HPT hsc_env) (moduleName breakInfo_module) of
    Just hmi -> modBreaks_locs (getModBreaks hmi) ! breakInfo_number
    _ -> panic "getHistorySpan"

{- | Finds the enclosing top level function name -}
-- ToDo: a better way to do this would be to keep hold of the decl_path computed
-- by the coverage pass, which gives the list of lexically-enclosing bindings
-- for each tick.
findEnclosingDecls :: HscEnv -> BreakInfo -> [String]
findEnclosingDecls hsc_env (BreakInfo modl ix) =
   let hmi = expectJust "findEnclosingDecls" $
             lookupHpt (hsc_HPT hsc_env) (moduleName modl)
       mb = getModBreaks hmi
   in modBreaks_decls mb ! ix

-- | Update fixity environment in the current interactive context.
updateFixityEnv :: GhcMonad m => FixityEnv -> m ()
updateFixityEnv fix_env = do
  hsc_env <- getSession
  let ic = hsc_IC hsc_env
  setSession $ hsc_env { hsc_IC = ic { ic_fix_env = fix_env } }

-- -----------------------------------------------------------------------------
-- execStmt

-- | default ExecOptions
execOptions :: ExecOptions
execOptions = ExecOptions
  { execSingleStep = RunToCompletion
  , execSourceFile = "<interactive>"
  , execLineNumber = 1
  , execWrap = EvalThis -- just run the statement, don't wrap it in anything
  }

-- | Run a statement in the current interactive context.
execStmt
  :: GhcMonad m
  => String             -- ^ a statement (bind or expression)
  -> ExecOptions
  -> m ExecResult
execStmt input exec_opts@ExecOptions{..} = do
    hsc_env <- getSession

    mb_stmt <-
      liftIO $
      runInteractiveHsc hsc_env $
      hscParseStmtWithLocation execSourceFile execLineNumber input

    case mb_stmt of
      -- empty statement / comment
      Nothing -> return (ExecComplete (Right []) 0)
      Just stmt -> execStmt' stmt input exec_opts

-- | Like `execStmt`, but takes a parsed statement as argument. Useful when
-- doing preprocessing on the AST before execution, e.g. in GHCi (see
-- GHCi.UI.runStmt).
execStmt' :: GhcMonad m => GhciLStmt GhcPs -> String -> ExecOptions -> m ExecResult
execStmt' stmt stmt_text ExecOptions{..} = do
    hsc_env <- getSession

    -- Turn off -fwarn-unused-local-binds when running a statement, to hide
    -- warnings about the implicit bindings we introduce.
    -- (This is basically `mkInteractiveHscEnv hsc_env`, except we unset
    -- -wwarn-unused-local-binds)
    let ic       = hsc_IC hsc_env -- use the interactive dflags
        idflags' = ic_dflags ic `wopt_unset` Opt_WarnUnusedLocalBinds
        hsc_env' = mkInteractiveHscEnv (hsc_env{ hsc_IC = ic{ ic_dflags = idflags' } })

    r <- liftIO $ hscParsedStmt hsc_env' stmt

    case r of
      Nothing ->
        -- empty statement / comment
        return (ExecComplete (Right []) 0)
      Just (ids, hval, fix_env) -> do
        updateFixityEnv fix_env

        status <-
          withVirtualCWD $
            liftIO $
              evalStmt hsc_env' (isStep execSingleStep) (execWrap hval)

        let ic = hsc_IC hsc_env
            bindings = (ic_tythings ic, ic_rn_gbl_env ic)

            size = ghciHistSize idflags'

        handleRunStatus execSingleStep stmt_text bindings ids
                        status (emptyHistory size)

runDecls :: GhcMonad m => String -> m [Name]
runDecls = runDeclsWithLocation "<interactive>" 1

-- | Run some declarations and return any user-visible names that were brought
-- into scope.
runDeclsWithLocation :: GhcMonad m => String -> Int -> String -> m [Name]
runDeclsWithLocation source line_num input = do
    hsc_env <- getSession
    decls <- liftIO (hscParseDeclsWithLocation hsc_env source line_num input)
    runParsedDecls decls

-- | Like `runDeclsWithLocation`, but takes parsed declarations as argument.
-- Useful when doing preprocessing on the AST before execution, e.g. in GHCi
-- (see GHCi.UI.runStmt).
runParsedDecls :: GhcMonad m => [LHsDecl GhcPs] -> m [Name]
runParsedDecls decls = do
    hsc_env <- getSession
    (tyThings, ic) <- liftIO (hscParsedDecls hsc_env decls)

    setSession $ hsc_env { hsc_IC = ic }
    hsc_env <- getSession
    hsc_env' <- liftIO $ rttiEnvironment hsc_env
    setSession hsc_env'
    return $ filter (not . isDerivedOccName . nameOccName)
             -- For this filter, see Note [What to show to users]
           $ map getName tyThings

{- Note [What to show to users]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We don't want to display internally-generated bindings to users.
Things like the coercion axiom for newtypes. These bindings all get
OccNames that users can't write, to avoid the possibility of name
clashes (in linker symbols).  That gives a convenient way to suppress
them. The relevant predicate is OccName.isDerivedOccName.
See #11051 for more background and examples.
-}

withVirtualCWD :: GhcMonad m => m a -> m a
withVirtualCWD m = do
  hsc_env <- getSession

    -- a virtual CWD is only necessary when we're running interpreted code in
    -- the same process as the compiler.
  case hsc_interp hsc_env of
    Just (ExternalInterp {}) -> m
    _ -> do
      let ic = hsc_IC hsc_env
      let set_cwd = do
            dir <- liftIO $ getCurrentDirectory
            case ic_cwd ic of
               Just dir -> liftIO $ setCurrentDirectory dir
               Nothing  -> return ()
            return dir

          reset_cwd orig_dir = do
            virt_dir <- liftIO $ getCurrentDirectory
            hsc_env <- getSession
            let old_IC = hsc_IC hsc_env
            setSession hsc_env{  hsc_IC = old_IC{ ic_cwd = Just virt_dir } }
            liftIO $ setCurrentDirectory orig_dir

      MC.bracket set_cwd reset_cwd $ \_ -> m

parseImportDecl :: GhcMonad m => String -> m (ImportDecl GhcPs)
parseImportDecl expr = withSession $ \hsc_env -> liftIO $ hscImport hsc_env expr

emptyHistory :: Int -> BoundedList History
emptyHistory size = nilBL size

handleRunStatus :: GhcMonad m
                => SingleStep -> String-> ([TyThing],GlobalRdrEnv) -> [Id]
                -> EvalStatus_ [ForeignHValue] [HValueRef]
                -> BoundedList History
                -> m ExecResult

handleRunStatus step expr bindings final_ids status history
  | RunAndLogSteps <- step = tracing
  | otherwise              = not_tracing
 where
  tracing
    | EvalBreak is_exception apStack_ref ix mod_uniq resume_ctxt _ccs <- status
    , not is_exception
    = do
       hsc_env <- getSession
       let hmi = expectJust "handleRunStatus" $
                   lookupHptDirectly (hsc_HPT hsc_env)
                                     (mkUniqueGrimily mod_uniq)
           modl = mi_module (hm_iface hmi)
           breaks = getModBreaks hmi

       b <- liftIO $
              breakpointStatus hsc_env (modBreaks_flags breaks) ix
       if b
         then not_tracing
           -- This breakpoint is explicitly enabled; we want to stop
           -- instead of just logging it.
         else do
           apStack_fhv <- liftIO $ mkFinalizedHValue hsc_env apStack_ref
           let bi = BreakInfo modl ix
               !history' = mkHistory hsc_env apStack_fhv bi `consBL` history
                 -- history is strict, otherwise our BoundedList is pointless.
           fhv <- liftIO $ mkFinalizedHValue hsc_env resume_ctxt
           status <- liftIO $ GHCi.resumeStmt hsc_env True fhv
           handleRunStatus RunAndLogSteps expr bindings final_ids
                           status history'
    | otherwise
    = not_tracing

  not_tracing
    -- Hit a breakpoint
    | EvalBreak is_exception apStack_ref ix mod_uniq resume_ctxt ccs <- status
    = do
         hsc_env <- getSession
         resume_ctxt_fhv <- liftIO $ mkFinalizedHValue hsc_env resume_ctxt
         apStack_fhv <- liftIO $ mkFinalizedHValue hsc_env apStack_ref
         let hmi = expectJust "handleRunStatus" $
                     lookupHptDirectly (hsc_HPT hsc_env)
                                       (mkUniqueGrimily mod_uniq)
             modl = mi_module (hm_iface hmi)
             bp | is_exception = Nothing
                | otherwise = Just (BreakInfo modl ix)
         (hsc_env1, names, span, decl) <- liftIO $
           bindLocalsAtBreakpoint hsc_env apStack_fhv bp
         let
           resume = Resume
             { resumeStmt = expr, resumeContext = resume_ctxt_fhv
             , resumeBindings = bindings, resumeFinalIds = final_ids
             , resumeApStack = apStack_fhv
             , resumeBreakInfo = bp
             , resumeSpan = span, resumeHistory = toListBL history
             , resumeDecl = decl
             , resumeCCS = ccs
             , resumeHistoryIx = 0 }
           hsc_env2 = pushResume hsc_env1 resume

         setSession hsc_env2
         return (ExecBreak names bp)

    -- Completed successfully
    | EvalComplete allocs (EvalSuccess hvals) <- status
    = do hsc_env <- getSession
         let final_ic = extendInteractiveContextWithIds (hsc_IC hsc_env) final_ids
             final_names = map getName final_ids
             dl = hsc_dynLinker hsc_env
         liftIO $ Linker.extendLinkEnv dl (zip final_names hvals)
         hsc_env' <- liftIO $ rttiEnvironment hsc_env{hsc_IC=final_ic}
         setSession hsc_env'
         return (ExecComplete (Right final_names) allocs)

    -- Completed with an exception
    | EvalComplete alloc (EvalException e) <- status
    = return (ExecComplete (Left (fromSerializableException e)) alloc)

#if __GLASGOW_HASKELL__ <= 810
    | otherwise
    = panic "not_tracing" -- actually exhaustive, but GHC can't tell
#endif


resumeExec :: GhcMonad m => (SrcSpan->Bool) -> SingleStep -> m ExecResult
resumeExec canLogSpan step
 = do
   hsc_env <- getSession
   let ic = hsc_IC hsc_env
       resume = ic_resume ic

   case resume of
     [] -> liftIO $
           throwGhcExceptionIO (ProgramError "not stopped at a breakpoint")
     (r:rs) -> do
        -- unbind the temporary locals by restoring the TypeEnv from
        -- before the breakpoint, and drop this Resume from the
        -- InteractiveContext.
        let (resume_tmp_te,resume_rdr_env) = resumeBindings r
            ic' = ic { ic_tythings = resume_tmp_te,
                       ic_rn_gbl_env = resume_rdr_env,
                       ic_resume   = rs }
        setSession hsc_env{ hsc_IC = ic' }

        -- remove any bindings created since the breakpoint from the
        -- linker's environment
        let old_names = map getName resume_tmp_te
            new_names = [ n | thing <- ic_tythings ic
                            , let n = getName thing
                            , not (n `elem` old_names) ]
            dl        = hsc_dynLinker hsc_env
        liftIO $ Linker.deleteFromLinkEnv dl new_names

        case r of
          Resume { resumeStmt = expr, resumeContext = fhv
                 , resumeBindings = bindings, resumeFinalIds = final_ids
                 , resumeApStack = apStack, resumeBreakInfo = mb_brkpt
                 , resumeSpan = span
                 , resumeHistory = hist } -> do
               withVirtualCWD $ do
                status <- liftIO $ GHCi.resumeStmt hsc_env (isStep step) fhv
                let prevHistoryLst = fromListBL 50 hist
                    hist' = case mb_brkpt of
                       Nothing -> prevHistoryLst
                       Just bi
                         | not $canLogSpan span -> prevHistoryLst
                         | otherwise -> mkHistory hsc_env apStack bi `consBL`
                                                        fromListBL 50 hist
                handleRunStatus step expr bindings final_ids status hist'

back :: GhcMonad m => Int -> m ([Name], Int, SrcSpan, String)
back n = moveHist (+n)

forward :: GhcMonad m => Int -> m ([Name], Int, SrcSpan, String)
forward n = moveHist (subtract n)

moveHist :: GhcMonad m => (Int -> Int) -> m ([Name], Int, SrcSpan, String)
moveHist fn = do
  hsc_env <- getSession
  case ic_resume (hsc_IC hsc_env) of
     [] -> liftIO $
           throwGhcExceptionIO (ProgramError "not stopped at a breakpoint")
     (r:rs) -> do
        let ix = resumeHistoryIx r
            history = resumeHistory r
            new_ix = fn ix
        --
        when (history `lengthLessThan` new_ix) $ liftIO $
           throwGhcExceptionIO (ProgramError "no more logged breakpoints")
        when (new_ix < 0) $ liftIO $
           throwGhcExceptionIO (ProgramError "already at the beginning of the history")

        let
          update_ic apStack mb_info = do
            (hsc_env1, names, span, decl) <-
              liftIO $ bindLocalsAtBreakpoint hsc_env apStack mb_info
            let ic = hsc_IC hsc_env1
                r' = r { resumeHistoryIx = new_ix }
                ic' = ic { ic_resume = r':rs }

            setSession hsc_env1{ hsc_IC = ic' }

            return (names, new_ix, span, decl)

        -- careful: we want apStack to be the AP_STACK itself, not a thunk
        -- around it, hence the cases are carefully constructed below to
        -- make this the case.  ToDo: this is v. fragile, do something better.
        if new_ix == 0
           then case r of
                   Resume { resumeApStack = apStack,
                            resumeBreakInfo = mb_brkpt } ->
                          update_ic apStack mb_brkpt
           else case history !! (new_ix - 1) of
                   History{..} ->
                     update_ic historyApStack (Just historyBreakInfo)


-- -----------------------------------------------------------------------------
-- After stopping at a breakpoint, add free variables to the environment

result_fs :: FastString
result_fs = fsLit "_result"

bindLocalsAtBreakpoint
        :: HscEnv
        -> ForeignHValue
        -> Maybe BreakInfo
        -> IO (HscEnv, [Name], SrcSpan, String)

-- Nothing case: we stopped when an exception was raised, not at a
-- breakpoint.  We have no location information or local variables to
-- bind, all we can do is bind a local variable to the exception
-- value.
bindLocalsAtBreakpoint hsc_env apStack Nothing = do
   let exn_occ = mkVarOccFS (fsLit "_exception")
       span    = mkGeneralSrcSpan (fsLit "<unknown>")
   exn_name <- newInteractiveBinder hsc_env exn_occ span

   let e_fs    = fsLit "e"
       e_name  = mkInternalName (getUnique e_fs) (mkTyVarOccFS e_fs) span
       e_tyvar = mkRuntimeUnkTyVar e_name liftedTypeKind
       exn_id  = Id.mkVanillaGlobal exn_name (mkTyVarTy e_tyvar)

       ictxt0 = hsc_IC hsc_env
       ictxt1 = extendInteractiveContextWithIds ictxt0 [exn_id]
       dl     = hsc_dynLinker hsc_env
   --
   Linker.extendLinkEnv dl [(exn_name, apStack)]
   return (hsc_env{ hsc_IC = ictxt1 }, [exn_name], span, "<exception thrown>")

-- Just case: we stopped at a breakpoint, we have information about the location
-- of the breakpoint and the free variables of the expression.
bindLocalsAtBreakpoint hsc_env apStack_fhv (Just BreakInfo{..}) = do
   let
       hmi       = expectJust "bindLocalsAtBreakpoint" $
                     lookupHpt (hsc_HPT hsc_env) (moduleName breakInfo_module)
       breaks    = getModBreaks hmi
       info      = expectJust "bindLocalsAtBreakpoint2" $
                     IntMap.lookup breakInfo_number (modBreaks_breakInfo breaks)
       mbVars    = cgb_vars info
       result_ty = cgb_resty info
       occs      = modBreaks_vars breaks ! breakInfo_number
       span      = modBreaks_locs breaks ! breakInfo_number
       decl      = intercalate "." $ modBreaks_decls breaks ! breakInfo_number

           -- Filter out any unboxed ids by changing them to Nothings;
           -- we can't bind these at the prompt
       mbPointers = nullUnboxed <$> mbVars

       (ids, offsets, occs') = syncOccs mbPointers occs

       free_tvs = tyCoVarsOfTypesList (result_ty:map idType ids)

   -- It might be that getIdValFromApStack fails, because the AP_STACK
   -- has been accidentally evaluated, or something else has gone wrong.
   -- So that we don't fall over in a heap when this happens, just don't
   -- bind any free variables instead, and we emit a warning.
   mb_hValues <-
      mapM (getBreakpointVar hsc_env apStack_fhv . fromIntegral) offsets
   when (any isNothing mb_hValues) $
      debugTraceMsg (hsc_dflags hsc_env) 1 $
          text "Warning: _result has been evaluated, some bindings have been lost"

   us <- mkSplitUniqSupply 'I'   -- Dodgy; will give the same uniques every time
   let tv_subst     = newTyVars us free_tvs
       (filtered_ids, occs'') = unzip         -- again, sync the occ-names
          [ (id, occ) | (id, Just _hv, occ) <- zip3 ids mb_hValues occs' ]
       (_,tidy_tys) = tidyOpenTypes emptyTidyEnv $
                      map (substTy tv_subst . idType) filtered_ids

   new_ids     <- zipWith3M mkNewId occs'' tidy_tys filtered_ids
   result_name <- newInteractiveBinder hsc_env (mkVarOccFS result_fs) span

   let result_id = Id.mkVanillaGlobal result_name
                     (substTy tv_subst result_ty)
       result_ok = isPointer result_id

       final_ids | result_ok = result_id : new_ids
                 | otherwise = new_ids
       ictxt0 = hsc_IC hsc_env
       ictxt1 = extendInteractiveContextWithIds ictxt0 final_ids
       names  = map idName new_ids
       dl     = hsc_dynLinker hsc_env

   let fhvs = catMaybes mb_hValues
   Linker.extendLinkEnv dl (zip names fhvs)
   when result_ok $ Linker.extendLinkEnv dl [(result_name, apStack_fhv)]
   hsc_env1 <- rttiEnvironment hsc_env{ hsc_IC = ictxt1 }
   return (hsc_env1, if result_ok then result_name:names else names, span, decl)
  where
        -- We need a fresh Unique for each Id we bind, because the linker
        -- state is single-threaded and otherwise we'd spam old bindings
        -- whenever we stop at a breakpoint.  The InteractveContext is properly
        -- saved/restored, but not the linker state.  See #1743, test break026.
   mkNewId :: OccName -> Type -> Id -> IO Id
   mkNewId occ ty old_id
     = do { name <- newInteractiveBinder hsc_env occ (getSrcSpan old_id)
          ; return (Id.mkVanillaGlobalWithInfo name ty (idInfo old_id)) }

   newTyVars :: UniqSupply -> [TcTyVar] -> TCvSubst
     -- Similarly, clone the type variables mentioned in the types
     -- we have here, *and* make them all RuntimeUnk tyvars
   newTyVars us tvs
     = mkTvSubstPrs [ (tv, mkTyVarTy (mkRuntimeUnkTyVar name (tyVarKind tv)))
                    | (tv, uniq) <- tvs `zip` uniqsFromSupply us
                    , let name = setNameUnique (tyVarName tv) uniq ]

   isPointer id | [rep] <- typePrimRep (idType id)
                , isGcPtrRep rep                   = True
                | otherwise                        = False

   -- Convert unboxed Id's to Nothings
   nullUnboxed (Just (fv@(id, _)))
     | isPointer id          = Just fv
     | otherwise             = Nothing
   nullUnboxed Nothing       = Nothing

   -- See Note [Syncing breakpoint info]
   syncOccs :: [Maybe (a,b)] -> [c] -> ([a], [b], [c])
   syncOccs mbVs ocs = unzip3 $ catMaybes $ joinOccs mbVs ocs
     where
       joinOccs :: [Maybe (a,b)] -> [c] -> [Maybe (a,b,c)]
       joinOccs = zipWithEqual "bindLocalsAtBreakpoint" joinOcc
       joinOcc mbV oc = (\(a,b) c -> (a,b,c)) <$> mbV <*> pure oc

rttiEnvironment :: HscEnv -> IO HscEnv
rttiEnvironment hsc_env@HscEnv{hsc_IC=ic} = do
   let tmp_ids = [id | AnId id <- ic_tythings ic]
       incompletelyTypedIds =
           [id | id <- tmp_ids
               , not $ noSkolems id
               , (occNameFS.nameOccName.idName) id /= result_fs]
   hsc_env' <- foldM improveTypes hsc_env (map idName incompletelyTypedIds)
   return hsc_env'
    where
     noSkolems = noFreeVarsOfType . idType
     improveTypes hsc_env@HscEnv{hsc_IC=ic} name = do
      let tmp_ids = [id | AnId id <- ic_tythings ic]
          Just id = find (\i -> idName i == name) tmp_ids
      if noSkolems id
         then return hsc_env
         else do
           mb_new_ty <- reconstructType hsc_env 10 id
           let old_ty = idType id
           case mb_new_ty of
             Nothing -> return hsc_env
             Just new_ty -> do
              case improveRTTIType hsc_env old_ty new_ty of
               Nothing -> return $
                        WARN(True, text (":print failed to calculate the "
                                           ++ "improvement for a type")) hsc_env
               Just subst -> do
                 let dflags = hsc_dflags hsc_env
                 dumpIfSet_dyn dflags Opt_D_dump_rtti "RTTI"
                   FormatText
                   (fsep [text "RTTI Improvement for", ppr id, equals,
                          ppr subst])

                 let ic' = substInteractiveContext ic subst
                 return hsc_env{hsc_IC=ic'}

pushResume :: HscEnv -> Resume -> HscEnv
pushResume hsc_env resume = hsc_env { hsc_IC = ictxt1 }
  where
        ictxt0 = hsc_IC hsc_env
        ictxt1 = ictxt0 { ic_resume = resume : ic_resume ictxt0 }


  {-
  Note [Syncing breakpoint info]

  To display the values of the free variables for a single breakpoint, the
  function `GHC.Runtime.Eval.bindLocalsAtBreakpoint` pulls
  out the information from the fields `modBreaks_breakInfo` and
  `modBreaks_vars` of the `ModBreaks` data structure.
  For a specific breakpoint this gives 2 lists of type `Id` (or `Var`)
  and `OccName`.
  They are used to create the Id's for the free variables and must be kept
  in sync!

  There are 3 situations where items are removed from the Id list
  (or replaced with `Nothing`):
  1.) If function `GHC.CoreToByteCode.schemeER_wrk` (which creates
      the Id list) doesn't find an Id in the ByteCode environement.
  2.) If function `GHC.Runtime.Eval.bindLocalsAtBreakpoint`
      filters out unboxed elements from the Id list, because GHCi cannot
      yet handle them.
  3.) If the GHCi interpreter doesn't find the reference to a free variable
      of our breakpoint. This also happens in the function
      bindLocalsAtBreakpoint.

  If an element is removed from the Id list, then the corresponding element
  must also be removed from the Occ list. Otherwise GHCi will confuse
  variable names as in #8487.
  -}

-- -----------------------------------------------------------------------------
-- Abandoning a resume context

abandon :: GhcMonad m => m Bool
abandon = do
   hsc_env <- getSession
   let ic = hsc_IC hsc_env
       resume = ic_resume ic
   case resume of
      []    -> return False
      r:rs  -> do
         setSession hsc_env{ hsc_IC = ic { ic_resume = rs } }
         liftIO $ abandonStmt hsc_env (resumeContext r)
         return True

abandonAll :: GhcMonad m => m Bool
abandonAll = do
   hsc_env <- getSession
   let ic = hsc_IC hsc_env
       resume = ic_resume ic
   case resume of
      []  -> return False
      rs  -> do
         setSession hsc_env{ hsc_IC = ic { ic_resume = [] } }
         liftIO $ mapM_ (abandonStmt hsc_env. resumeContext) rs
         return True

-- -----------------------------------------------------------------------------
-- Bounded list, optimised for repeated cons

data BoundedList a = BL
                        {-# UNPACK #-} !Int  -- length
                        {-# UNPACK #-} !Int  -- bound
                        [a] -- left
                        [a] -- right,  list is (left ++ reverse right)

nilBL :: Int -> BoundedList a
nilBL bound = BL 0 bound [] []

consBL :: a -> BoundedList a -> BoundedList a
consBL a (BL len bound left right)
  | len < bound = BL (len+1) bound (a:left) right
  | null right  = BL len     bound [a]      $! tail (reverse left)
  | otherwise   = BL len     bound (a:left) $! tail right

toListBL :: BoundedList a -> [a]
toListBL (BL _ _ left right) = left ++ reverse right

fromListBL :: Int -> [a] -> BoundedList a
fromListBL bound l = BL (length l) bound l []

-- lenBL (BL len _ _ _) = len

-- -----------------------------------------------------------------------------
-- | Set the interactive evaluation context.
--
-- (setContext imports) sets the ic_imports field (which in turn
-- determines what is in scope at the prompt) to 'imports', and
-- constructs the ic_rn_glb_env environment to reflect it.
--
-- We retain in scope all the things defined at the prompt, and kept
-- in ic_tythings.  (Indeed, they shadow stuff from ic_imports.)

setContext :: GhcMonad m => [InteractiveImport] -> m ()
setContext imports
  = do { hsc_env <- getSession
       ; let dflags = hsc_dflags hsc_env
       ; all_env_err <- liftIO $ findGlobalRdrEnv hsc_env imports
       ; case all_env_err of
           Left (mod, err) ->
               liftIO $ throwGhcExceptionIO (formatError dflags mod err)
           Right all_env -> do {
       ; let old_ic         = hsc_IC hsc_env
             !final_rdr_env = all_env `icExtendGblRdrEnv` ic_tythings old_ic
       ; setSession
         hsc_env{ hsc_IC = old_ic { ic_imports    = imports
                                  , ic_rn_gbl_env = final_rdr_env }}}}
  where
    formatError dflags mod err = ProgramError . showSDoc dflags $
      text "Cannot add module" <+> ppr mod <+>
      text "to context:" <+> text err

findGlobalRdrEnv :: HscEnv -> [InteractiveImport]
                 -> IO (Either (ModuleName, String) GlobalRdrEnv)
-- Compute the GlobalRdrEnv for the interactive context
findGlobalRdrEnv hsc_env imports
  = do { idecls_env <- hscRnImportDecls hsc_env idecls
                    -- This call also loads any orphan modules
       ; return $ case partitionEithers (map mkEnv imods) of
           ([], imods_env) -> Right (foldr plusGlobalRdrEnv idecls_env imods_env)
           (err : _, _)    -> Left err }
  where
    idecls :: [LImportDecl GhcPs]
    idecls = [noLoc d | IIDecl d <- imports]

    imods :: [ModuleName]
    imods = [m | IIModule m <- imports]

    mkEnv mod = case mkTopLevEnv (hsc_HPT hsc_env) mod of
      Left err -> Left (mod, err)
      Right env -> Right env

availsToGlobalRdrEnv :: ModuleName -> [AvailInfo] -> GlobalRdrEnv
availsToGlobalRdrEnv mod_name avails
  = mkGlobalRdrEnv (gresFromAvails (Just imp_spec) avails)
  where
      -- We're building a GlobalRdrEnv as if the user imported
      -- all the specified modules into the global interactive module
    imp_spec = ImpSpec { is_decl = decl, is_item = ImpAll}
    decl = ImpDeclSpec { is_mod = mod_name, is_as = mod_name,
                         is_qual = False,
                         is_dloc = srcLocSpan interactiveSrcLoc }

mkTopLevEnv :: HomePackageTable -> ModuleName -> Either String GlobalRdrEnv
mkTopLevEnv hpt modl
  = case lookupHpt hpt modl of
      Nothing -> Left "not a home module"
      Just details ->
         case mi_globals (hm_iface details) of
                Nothing  -> Left "not interpreted"
                Just env -> Right env

-- | Get the interactive evaluation context, consisting of a pair of the
-- set of modules from which we take the full top-level scope, and the set
-- of modules from which we take just the exports respectively.
getContext :: GhcMonad m => m [InteractiveImport]
getContext = withSession $ \HscEnv{ hsc_IC=ic } ->
             return (ic_imports ic)

-- | Returns @True@ if the specified module is interpreted, and hence has
-- its full top-level scope available.
moduleIsInterpreted :: GhcMonad m => Module -> m Bool
moduleIsInterpreted modl = withSession $ \h ->
 if not (isHomeModule (hsc_dflags h) modl)
        then return False
        else case lookupHpt (hsc_HPT h) (moduleName modl) of
                Just details       -> return (isJust (mi_globals (hm_iface details)))
                _not_a_home_module -> return False

-- | Looks up an identifier in the current interactive context (for :info)
-- Filter the instances by the ones whose tycons (or clases resp)
-- are in scope (qualified or otherwise).  Otherwise we list a whole lot too many!
-- The exact choice of which ones to show, and which to hide, is a judgement call.
--      (see #1581)
getInfo :: GhcMonad m => Bool -> Name
        -> m (Maybe (TyThing,Fixity,[ClsInst],[FamInst], SDoc))
getInfo allInfo name
  = withSession $ \hsc_env ->
    do mb_stuff <- liftIO $ hscTcRnGetInfo hsc_env name
       case mb_stuff of
         Nothing -> return Nothing
         Just (thing, fixity, cls_insts, fam_insts, docs) -> do
           let rdr_env = ic_rn_gbl_env (hsc_IC hsc_env)

           -- Filter the instances based on whether the constituent names of their
           -- instance heads are all in scope.
           let cls_insts' = filter (plausible rdr_env . orphNamesOfClsInst) cls_insts
               fam_insts' = filter (plausible rdr_env . orphNamesOfFamInst) fam_insts
           return (Just (thing, fixity, cls_insts', fam_insts', docs))
  where
    plausible rdr_env names
          -- Dfun involving only names that are in ic_rn_glb_env
        = allInfo
       || nameSetAll ok names
        where   -- A name is ok if it's in the rdr_env,
                -- whether qualified or not
          ok n | n == name              = True
                       -- The one we looked for in the first place!
               | pretendNameIsInScope n = True
               | isBuiltInSyntax n      = True
               | isCTupleTyConName n    = True
               | isExternalName n       = isJust (lookupGRE_Name rdr_env n)
               | otherwise              = True

-- | Returns all names in scope in the current interactive context
getNamesInScope :: GhcMonad m => m [Name]
getNamesInScope = withSession $ \hsc_env -> do
  return (map gre_name (globalRdrEnvElts (ic_rn_gbl_env (hsc_IC hsc_env))))

-- | Returns all 'RdrName's in scope in the current interactive
-- context, excluding any that are internally-generated.
getRdrNamesInScope :: GhcMonad m => m [RdrName]
getRdrNamesInScope = withSession $ \hsc_env -> do
  let
      ic = hsc_IC hsc_env
      gbl_rdrenv = ic_rn_gbl_env ic
      gbl_names = concatMap greRdrNames $ globalRdrEnvElts gbl_rdrenv
  -- Exclude internally generated names; see e.g. #11328
  return (filter (not . isDerivedOccName . rdrNameOcc) gbl_names)


-- | Parses a string as an identifier, and returns the list of 'Name's that
-- the identifier can refer to in the current interactive context.
parseName :: GhcMonad m => String -> m [Name]
parseName str = withSession $ \hsc_env -> liftIO $
   do { lrdr_name <- hscParseIdentifier hsc_env str
      ; hscTcRnLookupRdrName hsc_env lrdr_name }

-- | Returns @True@ if passed string is a statement.
isStmt :: ParserFlags -> String -> Bool
isStmt pflags stmt =
  case parseThing Parser.parseStmt pflags stmt of
    Lexer.POk _ _ -> True
    Lexer.PFailed _ -> False

-- | Returns @True@ if passed string has an import declaration.
hasImport :: ParserFlags -> String -> Bool
hasImport pflags stmt =
  case parseThing Parser.parseModule pflags stmt of
    Lexer.POk _ thing -> hasImports thing
    Lexer.PFailed _ -> False
  where
    hasImports = not . null . hsmodImports . unLoc

-- | Returns @True@ if passed string is an import declaration.
isImport :: ParserFlags -> String -> Bool
isImport pflags stmt =
  case parseThing Parser.parseImport pflags stmt of
    Lexer.POk _ _ -> True
    Lexer.PFailed _ -> False

-- | Returns @True@ if passed string is a declaration but __/not a splice/__.
isDecl :: ParserFlags -> String -> Bool
isDecl pflags stmt = do
  case parseThing Parser.parseDeclaration pflags stmt of
    Lexer.POk _ thing ->
      case unLoc thing of
        SpliceD _ _ -> False
        _ -> True
    Lexer.PFailed _ -> False

parseThing :: Lexer.P thing -> ParserFlags -> String -> Lexer.ParseResult thing
parseThing parser pflags stmt = do
  let buf = stringToStringBuffer stmt
      loc = mkRealSrcLoc (fsLit "<interactive>") 1 1

  Lexer.unP parser (Lexer.mkPStatePure pflags buf loc)

getDocs :: GhcMonad m
        => Name
        -> m (Either GetDocsFailure (Maybe HsDocString, Map Int HsDocString))
           -- TODO: What about docs for constructors etc.?
getDocs name =
  withSession $ \hsc_env -> do
     case nameModule_maybe name of
       Nothing -> pure (Left (NameHasNoModule name))
       Just mod -> do
         if isInteractiveModule mod
           then pure (Left InteractiveName)
           else do
             ModIface { mi_doc_hdr = mb_doc_hdr
                      , mi_decl_docs = DeclDocMap dmap
                      , mi_arg_docs = ArgDocMap amap
                      } <- liftIO $ hscGetModuleInterface hsc_env mod
             if isNothing mb_doc_hdr && Map.null dmap && Map.null amap
               then pure (Left (NoDocsInIface mod compiled))
               else pure (Right ( Map.lookup name dmap
                                , Map.findWithDefault Map.empty name amap))
  where
    compiled =
      -- TODO: Find a more direct indicator.
      case nameSrcLoc name of
        RealSrcLoc {} -> False
        UnhelpfulLoc {} -> True

-- | Failure modes for 'getDocs'.

-- TODO: Find a way to differentiate between modules loaded without '-haddock'
-- and modules that contain no docs.
data GetDocsFailure

    -- | 'nameModule_maybe' returned 'Nothing'.
  = NameHasNoModule Name

    -- | This is probably because the module was loaded without @-haddock@,
    -- but it's also possible that the entire module contains no documentation.
  | NoDocsInIface
      Module
      Bool -- ^ 'True': The module was compiled.
           -- 'False': The module was :loaded.

    -- | The 'Name' was defined interactively.
  | InteractiveName

instance Outputable GetDocsFailure where
  ppr (NameHasNoModule name) =
    quotes (ppr name) <+> text "has no module where we could look for docs."
  ppr (NoDocsInIface mod compiled) = vcat
    [ text "Can't find any documentation for" <+> ppr mod <> char '.'
    , text "This is probably because the module was"
        <+> text (if compiled then "compiled" else "loaded")
        <+> text "without '-haddock',"
    , text "but it's also possible that the module contains no documentation."
    , text ""
    , if compiled
        then text "Try re-compiling with '-haddock'."
        else text "Try running ':set -haddock' and :load the file again."
        -- TODO: Figure out why :reload doesn't load the docs and maybe fix it.
    ]
  ppr InteractiveName =
    text "Docs are unavailable for interactive declarations."

-- -----------------------------------------------------------------------------
-- Getting the type of an expression

-- | Get the type of an expression
-- Returns the type as described by 'TcRnExprMode'
exprType :: GhcMonad m => TcRnExprMode -> String -> m Type
exprType mode expr = withSession $ \hsc_env -> do
   ty <- liftIO $ hscTcExpr hsc_env mode expr
   return $ tidyType emptyTidyEnv ty

-- -----------------------------------------------------------------------------
-- Getting the kind of a type

-- | Get the kind of a  type
typeKind  :: GhcMonad m => Bool -> String -> m (Type, Kind)
typeKind normalise str = withSession $ \hsc_env -> do
   liftIO $ hscKcType hsc_env normalise str

-- ----------------------------------------------------------------------------
-- Getting the class instances for a type

{-
  Note [Querying instances for a type]

  Here is the implementation of GHC proposal 41.
  (https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0041-ghci-instances.rst)

  The objective is to take a query string representing a (partial) type, and
  report all the class single-parameter class instances available to that type.
  Extending this feature to multi-parameter typeclasses is left as future work.

  The general outline of how we solve this is:

  1. Parse the type, leaving skolems in the place of type-holes.
  2. For every class, get a list of all instances that match with the query type.
  3. For every matching instance, ask GHC for the context the instance dictionary needs.
  4. Format and present the results, substituting our query into the instance
     and simplifying the context.

  For example, given the query "Maybe Int", we want to return:

  instance Show (Maybe Int)
  instance Read (Maybe Int)
  instance Eq   (Maybe Int)
  ....

  [Holes in queries]

  Often times we want to know what instances are available for a polymorphic type,
  like `Maybe a`, and we'd like to return instances such as:

  instance Show a => Show (Maybe a)
  ....

  These queries are expressed using type holes, so instead of `Maybe a` the user writes
  `Maybe _`, we parse the type and during zonking, we skolemise it, replacing the holes
  with (un-named) type variables.

  When zonking the type holes we have two real choices: replace them with Any or replace
  them with skolem typevars. Using skolem type variables ensures that the output is more
  intuitive to end users, and there is no difference in the results between Any and skolems.

-}

-- Find all instances that match a provided type
getInstancesForType :: GhcMonad m => Type -> m [ClsInst]
getInstancesForType ty = withSession $ \hsc_env -> do
  liftIO $ runInteractiveHsc hsc_env $ do
    ioMsgMaybe $ runTcInteractive hsc_env $ do
      -- Bring class and instances from unqualified modules into scope, this fixes #16793.
      loadUnqualIfaces hsc_env (hsc_IC hsc_env)
      matches <- findMatchingInstances ty

      fmap catMaybes . forM matches $ uncurry checkForExistence

-- Parse a type string and turn any holes into skolems
parseInstanceHead :: GhcMonad m => String -> m Type
parseInstanceHead str = withSession $ \hsc_env0 -> do
  (ty, _) <- liftIO $ runInteractiveHsc hsc_env0 $ do
    hsc_env <- getHscEnv
    ty <- hscParseType str
    ioMsgMaybe $ tcRnType hsc_env SkolemiseFlexi True ty

  return ty

-- Get all the constraints required of a dictionary binding
getDictionaryBindings :: PredType -> TcM CtEvidence
getDictionaryBindings theta = do
  dictName <- newName (mkDictOcc (mkVarOcc "magic"))
  let dict_var = mkVanillaGlobal dictName theta
  loc <- getCtLocM (GivenOrigin UnkSkol) Nothing

  -- Generate a wanted here because at the end of constraint
  -- solving, most derived constraints get thrown away, which in certain
  -- cases, notably with quantified constraints makes it impossible to rule
  -- out instances as invalid. (See #18071)
  return CtWanted {
    ctev_pred = varType dict_var,
    ctev_dest = EvVarDest dict_var,
    ctev_nosh = WDeriv,
    ctev_loc = loc
  }

-- Find instances where the head unifies with the provided type
findMatchingInstances :: Type -> TcM [(ClsInst, [DFunInstType])]
findMatchingInstances ty = do
  ies@(InstEnvs {ie_global = ie_global, ie_local = ie_local}) <- tcGetInstEnvs
  let allClasses = instEnvClasses ie_global ++ instEnvClasses ie_local
  return $ concatMap (try_cls ies) allClasses
  where
  {- Check that a class instance is well-kinded.
    Since `:instances` only works for unary classes, we're looking for instances of kind
    k -> Constraint where k is the type of the queried type.
  -}
  try_cls ies cls
    | Just (_, arg_kind, res_kind) <- splitFunTy_maybe (tyConKind $ classTyCon cls)
    , tcIsConstraintKind res_kind
    , Type.typeKind ty `eqType` arg_kind
    , (matches, _, _) <- lookupInstEnv True ies cls [ty]
    = matches
    | otherwise
    = []


{-
  When we've found an instance that a query matches against, we still need to
  check that all the instance's constraints are satisfiable. checkForExistence
  creates an instance dictionary and verifies that any unsolved constraints
  mention a type-hole, meaning it is blocked on an unknown.

  If the instance satisfies this condition, then we return it with the query
  substituted into the instance and all constraints simplified, for example given:

  instance D a => C (MyType a b) where

  and the query `MyType _ String`

  the unsolved constraints will be [D _] so we apply the substitution:

  { a -> _; b -> String}

  and return the instance:

  instance D _ => C (MyType _ String)

-}

checkForExistence :: ClsInst -> [DFunInstType] -> TcM (Maybe ClsInst)
checkForExistence clsInst mb_inst_tys = do
  -- We want to force the solver to attempt to solve the constraints for clsInst.
  -- Usually, this isn't a problem since there should only be a single instance
  -- for a type. However, when we have overlapping instances, the solver will give up
  -- since it can't decide which instance to use. To get around this restriction, instead
  -- of asking the solver to solve a constraint for clsInst, we ask it to solve the
  -- thetas of clsInst.
  (tys, thetas) <- instDFunType (is_dfun clsInst) mb_inst_tys
  wanteds <- mapM getDictionaryBindings thetas
  -- It's important to zonk constraints after solving in order to expose things like TypeErrors
  -- which otherwise appear as opaque type variables. (See #18262).
  WC { wc_simple = simples, wc_impl = impls } <- simplifyWantedsTcM wanteds

  if allBag allowedSimple simples && solvedImplics impls
  then return . Just $ substInstArgs tys (bagToList (mapBag ctPred simples)) clsInst
  else return Nothing

  where
  allowedSimple :: Ct -> Bool
  allowedSimple ct = isSatisfiablePred (ctPred ct)

  solvedImplics :: Bag Implication -> Bool
  solvedImplics impls = allBag (isSolvedStatus . ic_status) impls

  -- Stricter version of isTyVarClassPred that requires all TyConApps to have at least
  -- one argument or for the head to be a TyVar. The reason is that we want to ensure
  -- that all residual constraints mention a type-hole somewhere in the constraint,
  -- meaning that with the correct choice of a concrete type it could be possible for
  -- the constraint to be discharged.
  isSatisfiablePred :: PredType -> Bool
  isSatisfiablePred ty = case getClassPredTys_maybe ty of
      Just (_, tys@(_:_)) -> all isTyVarTy tys
      _                   -> isTyVarTy ty

  empty_subst = mkEmptyTCvSubst (mkInScopeSet (tyCoVarsOfType (idType $ is_dfun clsInst)))

  {- Create a ClsInst with instantiated arguments and constraints.

     The thetas are the list of constraints that couldn't be solved because
     they mention a type-hole.
  -}
  substInstArgs ::  [Type] -> [PredType] -> ClsInst -> ClsInst
  substInstArgs tys thetas inst = let
      subst = foldl' (\a b -> uncurry (extendTvSubstAndInScope a) b) empty_subst (zip dfun_tvs tys)
      -- Build instance head with arguments substituted in
      tau   = mkClassPred cls (substTheta subst args)
      -- Constrain the instance with any residual constraints
      phi   = mkPhiTy thetas tau
      sigma = mkForAllTys (map (\v -> Bndr v Inferred) dfun_tvs) phi

    in inst { is_dfun = (is_dfun inst) { varType = sigma }}
    where
    (dfun_tvs, _, cls, args) = instanceSig inst

-----------------------------------------------------------------------------
-- Compile an expression, run it, and deliver the result

-- | Parse an expression, the parsed expression can be further processed and
-- passed to compileParsedExpr.
parseExpr :: GhcMonad m => String -> m (LHsExpr GhcPs)
parseExpr expr = withSession $ \hsc_env -> do
  liftIO $ runInteractiveHsc hsc_env $ hscParseExpr expr

-- | Compile an expression, run it, and deliver the resulting HValue.
compileExpr :: GhcMonad m => String -> m HValue
compileExpr expr = do
  parsed_expr <- parseExpr expr
  compileParsedExpr parsed_expr

-- | Compile an expression, run it, and deliver the resulting HValue.
compileExprRemote :: GhcMonad m => String -> m ForeignHValue
compileExprRemote expr = do
  parsed_expr <- parseExpr expr
  compileParsedExprRemote parsed_expr

-- | Compile a parsed expression (before renaming), run it, and deliver
-- the resulting HValue.
compileParsedExprRemote :: GhcMonad m => LHsExpr GhcPs -> m ForeignHValue
compileParsedExprRemote expr@(L loc _) = withSession $ \hsc_env -> do
  -- > let _compileParsedExpr = expr
  -- Create let stmt from expr to make hscParsedStmt happy.
  -- We will ignore the returned [Id], namely [expr_id], and not really
  -- create a new binding.
  let expr_fs = fsLit "_compileParsedExpr"
      expr_name = mkInternalName (getUnique expr_fs) (mkTyVarOccFS expr_fs) loc
      let_stmt = L loc . LetStmt noExtField . L loc . (HsValBinds noExtField) $
        ValBinds noExtField
                     (unitBag $ mkHsVarBind loc (getRdrName expr_name) expr) []

  pstmt <- liftIO $ hscParsedStmt hsc_env let_stmt
  let (hvals_io, fix_env) = case pstmt of
        Just ([_id], hvals_io', fix_env') -> (hvals_io', fix_env')
        _ -> panic "compileParsedExprRemote"

  updateFixityEnv fix_env
  status <- liftIO $ evalStmt hsc_env False (EvalThis hvals_io)
  case status of
    EvalComplete _ (EvalSuccess [hval]) -> return hval
    EvalComplete _ (EvalException e) ->
      liftIO $ throwIO (fromSerializableException e)
    _ -> panic "compileParsedExpr"

compileParsedExpr :: GhcMonad m => LHsExpr GhcPs -> m HValue
compileParsedExpr expr = do
   fhv <- compileParsedExprRemote expr
   hsc_env <- getSession
   liftIO $ withInterp hsc_env $ \interp ->
      wormhole interp fhv

-- | Compile an expression, run it and return the result as a Dynamic.
dynCompileExpr :: GhcMonad m => String -> m Dynamic
dynCompileExpr expr = do
  parsed_expr <- parseExpr expr
  -- > Data.Dynamic.toDyn expr
  let loc = getLoc parsed_expr
      to_dyn_expr = mkHsApp (L loc . HsVar noExtField . L loc $ getRdrName toDynName)
                            parsed_expr
  hval <- compileParsedExpr to_dyn_expr
  return (unsafeCoerce hval :: Dynamic)

-----------------------------------------------------------------------------
-- show a module and it's source/object filenames

showModule :: GhcMonad m => ModSummary -> m String
showModule mod_summary =
    withSession $ \hsc_env -> do
        interpreted <- moduleIsBootOrNotObjectLinkable mod_summary
        let dflags = hsc_dflags hsc_env
        return (showModMsg dflags interpreted mod_summary)

moduleIsBootOrNotObjectLinkable :: GhcMonad m => ModSummary -> m Bool
moduleIsBootOrNotObjectLinkable mod_summary = withSession $ \hsc_env ->
  case lookupHpt (hsc_HPT hsc_env) (ms_mod_name mod_summary) of
        Nothing       -> panic "missing linkable"
        Just mod_info -> return $ case hm_linkable mod_info of
          Nothing       -> True
          Just linkable -> not (isObjectLinkable linkable)

----------------------------------------------------------------------------
-- RTTI primitives

obtainTermFromVal :: HscEnv -> Int -> Bool -> Type -> a -> IO Term
#if defined(HAVE_INTERNAL_INTERPRETER)
obtainTermFromVal hsc_env bound force ty x = withInterp hsc_env $ \case
  InternalInterp    -> cvObtainTerm hsc_env bound force ty (unsafeCoerce x)
#else
obtainTermFromVal hsc_env _bound _force _ty _x = withInterp hsc_env $ \case
#endif
  ExternalInterp {} -> throwIO (InstallationError
                        "this operation requires -fno-external-interpreter")

obtainTermFromId :: HscEnv -> Int -> Bool -> Id -> IO Term
obtainTermFromId hsc_env bound force id =  do
  hv <- Linker.getHValue hsc_env (varName id)
  cvObtainTerm (updEnv hsc_env) bound force (idType id) hv
 where updEnv env = env {hsc_dflags =                             -- #14828
            xopt_set (hsc_dflags env) ImpredicativeTypes}
         -- See Note [Setting ImpredicativeTypes for :print command]

-- Uses RTTI to reconstruct the type of an Id, making it less polymorphic
reconstructType :: HscEnv -> Int -> Id -> IO (Maybe Type)
reconstructType hsc_env bound id = do
  hv <- Linker.getHValue hsc_env (varName id)
  cvReconstructType hsc_env bound (idType id) hv

mkRuntimeUnkTyVar :: Name -> Kind -> TyVar
mkRuntimeUnkTyVar name kind = mkTcTyVar name kind RuntimeUnk


{-
Note [Setting ImpredicativeTypes for :print command]

If ImpredicativeTypes is not enabled, then `:print <term>` will fail if the
type of <term> has nested `forall`s or `=>`s.
This is because the GHCi debugger's internals will attempt to unify a
metavariable with the type of <term> and then display the result, but if the
type has nested `forall`s or `=>`s, then unification will fail.
As a result, `:print` will bail out and the unhelpful result will be
`<term> = (_t1::t1)` (where `t1` is a metavariable).

Beware: <term> can have nested `forall`s even if its definition doesn't use
RankNTypes! Here is an example from #14828:

  class Functor f where
    fmap :: (a -> b) -> f a -> f b

Somewhat surprisingly, `:print fmap` considers the type of fmap to have
nested foralls. This is because the GHCi debugger sees the type
`fmap :: forall f. Functor f => forall a b. (a -> b) -> f a -> f b`.
We could envision deeply instantiating this type to get the type
`forall f a b. Functor f => (a -> b) -> f a -> f b`,
but this trick wouldn't work for higher-rank types.

Instead, we adopt a simpler fix: enable `ImpredicativeTypes` when using
`:print` and friends in the GHCi debugger. This allows metavariables
to unify with types that have nested (or higher-rank) `forall`s/`=>`s,
which makes `:print fmap` display as
`fmap = (_t1::forall a b. Functor f => (a -> b) -> f a -> f b)`, as expected.

Although ImpredicativeTypes is a somewhat unpredictable from a type inference
perspective, there is no danger in using it in the GHCi debugger, since all
of the terms that the GHCi debugger deals with have already been typechecked.
-}