CompManager.lhs 42.7 KB
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%
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% (c) The University of Glasgow, 2002
%
% The Compilation Manager
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%
\begin{code}
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{-# OPTIONS -fvia-C #-}
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module CompManager ( 
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    ModuleGraph, 

    CmState, emptyCmState,  -- abstract
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    cmInit, 	   -- :: GhciMode -> IO CmState
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    cmDepAnal,	   -- :: CmState -> DynFlags -> [FilePath] -> IO ModuleGraph
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    cmLoadModules, -- :: CmState -> DynFlags -> ModuleGraph
		   --	 -> IO (CmState, [String])
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    cmUnload,	   -- :: CmState -> DynFlags -> IO CmState

    cmSetContext,  -- :: CmState -> String -> IO CmState

    cmGetContext,  -- :: CmState -> IO String
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#ifdef GHCI
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    cmInfoThing,   -- :: CmState -> DynFlags -> String -> IO (Maybe TyThing)
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    CmRunResult(..),
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    cmRunStmt,	   -- :: CmState -> DynFlags -> String
		   --	 -> IO (CmState, CmRunResult)
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    cmTypeOfExpr,  -- :: CmState -> DynFlags -> String
		   -- 	-> IO (CmState, Maybe String)
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    cmTypeOfName,  -- :: CmState -> Name -> IO (Maybe String)
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    cmCompileExpr, -- :: CmState -> DynFlags -> String 
		   -- 	-> IO (CmState, Maybe HValue)#endif
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#endif
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  )
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where

#include "HsVersions.h"

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import CmLink
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import CmTypes
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import DriverPipeline
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import DriverState	( v_Output_file )
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import DriverPhases
import DriverUtil
import Finder
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#ifdef GHCI
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import HscMain		( initPersistentCompilerState, hscThing )
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#else
import HscMain		( initPersistentCompilerState )
#endif
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import HscTypes
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import Name		( Name, NamedThing(..), nameRdrName, nameModule,
			  isHomePackageName )
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import RdrName		( emptyRdrEnv )
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import Module
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import GetImports
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import UniqFM
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import Unique		( Uniquable )
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import Digraph		( SCC(..), stronglyConnComp, flattenSCC, flattenSCCs )
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import ErrUtils		( showPass )
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import SysTools		( cleanTempFilesExcept )
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import Util
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import Outputable
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import Panic
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import CmdLineOpts	( DynFlags(..), getDynFlags )
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import IOExts
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#ifdef GHCI
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import RdrName		( lookupRdrEnv )
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import Id		( idType, idName )
import NameEnv
import Type		( tidyType )
import VarEnv		( emptyTidyEnv )
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import RnEnv		( unQualInScope, mkIfaceGlobalRdrEnv )
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import BasicTypes	( Fixity, defaultFixity )
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import Interpreter	( HValue )
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import HscMain		( hscStmt )
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import PrelGHC		( unsafeCoerce# )

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import Foreign
import CForeign
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import Exception	( Exception, try )
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#endif

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-- lang
import Exception	( throwDyn )
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-- std
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import Directory        ( getModificationTime, doesFileExist )
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import IO
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import Monad
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import List		( nub )
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import Maybe
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\end{code}
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\begin{code}
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-- Persistent state for the entire system
data CmState
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   = CmState {
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        hst   :: HomeSymbolTable,    -- home symbol table
        hit   :: HomeIfaceTable,     -- home interface table
        ui    :: UnlinkedImage,      -- the unlinked images
        mg    :: ModuleGraph,        -- the module graph
        gmode :: GhciMode,           -- NEVER CHANGES
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	ic    :: InteractiveContext, -- command-line binding info
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        pcs    :: PersistentCompilerState, -- compile's persistent state
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        pls    :: PersistentLinkerState    -- link's persistent state
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     }
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emptyCmState :: GhciMode -> Module -> IO CmState
emptyCmState gmode mod
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    = do pcs     <- initPersistentCompilerState
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         pls     <- emptyPLS
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         return (CmState { hst    = emptySymbolTable,
                           hit    = emptyIfaceTable,
                           ui     = emptyUI,
                           mg     = emptyMG, 
                           gmode  = gmode,
			   ic     = emptyInteractiveContext mod,
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                           pcs    = pcs,
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                           pls    = pls })
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emptyInteractiveContext mod
  = InteractiveContext { ic_module = mod, 
			 ic_rn_env = emptyRdrEnv,
			 ic_type_env = emptyTypeEnv }

defaultCurrentModuleName = mkModuleName "Prelude"
GLOBAL_VAR(defaultCurrentModule, error "no defaultCurrentModule", Module)

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-- CM internal types
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type UnlinkedImage = [Linkable]	-- the unlinked images (should be a set, really)
emptyUI :: UnlinkedImage
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emptyUI = []

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type ModuleGraph = [ModSummary]  -- the module graph, topologically sorted
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emptyMG :: ModuleGraph
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emptyMG = []
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-----------------------------------------------------------------------------
-- Produce an initial CmState.

cmInit :: GhciMode -> IO CmState
cmInit mode = do
   prel <- moduleNameToModule defaultCurrentModuleName
   writeIORef defaultCurrentModule prel
   emptyCmState mode prel
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-----------------------------------------------------------------------------
-- Setting the context doesn't throw away any bindings; the bindings
-- we've built up in the InteractiveContext simply move to the new
-- module.  They always shadow anything in scope in the current context.

cmSetContext :: CmState -> String -> IO CmState
cmSetContext cmstate str
   = do let mn = mkModuleName str
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	    modules_loaded = [ (name_of_summary s, ms_mod s) | s <- mg cmstate ]
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        m <- case lookup mn modules_loaded of
		Just m  -> return m
		Nothing -> do
		   mod <- moduleNameToModule mn
		   if isHomeModule mod 
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			then throwDyn (CmdLineError (showSDoc 
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				(quotes (ppr (moduleName mod))
 				  <+> text "is not currently loaded")))
		   	else return mod

	return cmstate{ ic = (ic cmstate){ic_module=m} }
		
cmGetContext :: CmState -> IO String
cmGetContext cmstate = return (moduleUserString (ic_module (ic cmstate)))

moduleNameToModule :: ModuleName -> IO Module
moduleNameToModule mn
 = do maybe_stuff <- findModule mn
      case maybe_stuff of
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	Nothing -> throwDyn (CmdLineError ("can't find module `"
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				    ++ moduleNameUserString mn ++ "'"))
	Just (m,_) -> return m

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-----------------------------------------------------------------------------
-- cmInfoThing: convert a String to a TyThing

-- A string may refer to more than one TyThing (eg. a constructor,
-- and type constructor), so we return a list of all the possible TyThings.

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#ifdef GHCI
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cmInfoThing :: CmState -> DynFlags -> String 
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	-> IO (CmState, PrintUnqualified, [(TyThing,Fixity)])
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cmInfoThing cmstate dflags id
   = do (new_pcs, things) <- hscThing dflags hst hit pcs icontext id
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	let pairs = map (\x -> (x, getFixity new_pcs (getName x))) things
	return (cmstate{ pcs=new_pcs }, unqual, pairs)
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   where 
     CmState{ hst=hst, hit=hit, pcs=pcs, pls=pls, ic=icontext } = cmstate
     unqual = getUnqual pcs hit icontext
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     getFixity :: PersistentCompilerState -> Name -> Fixity
     getFixity pcs name
	| Just iface  <- lookupModuleEnv iface_table (nameModule name),
	  Just fixity <- lookupNameEnv (mi_fixities iface) name
	  = fixity
	| otherwise
	  = defaultFixity
	where iface_table | isHomePackageName name = hit
			  | otherwise              = pcs_PIT pcs
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#endif
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-----------------------------------------------------------------------------
-- cmRunStmt:  Run a statement/expr.

#ifdef GHCI
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data CmRunResult
  = CmRunOk [Name] 		-- names bound by this evaluation
  | CmRunFailed 
  | CmRunDeadlocked		-- statement deadlocked
  | CmRunException Exception	-- statement raised an exception

cmRunStmt :: CmState -> DynFlags -> String -> IO (CmState, CmRunResult)		
cmRunStmt cmstate@CmState{ hst=hst, hit=hit, pcs=pcs, pls=pls, ic=icontext }
          dflags expr
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   = do 
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	let InteractiveContext { 
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	       	ic_rn_env = rn_env, 
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	       	ic_type_env = type_env } = icontext
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        (new_pcs, maybe_stuff) 
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	    <- hscStmt dflags hst hit pcs icontext expr False{-stmt-}
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        case maybe_stuff of
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	   Nothing -> return (cmstate{ pcs=new_pcs }, CmRunFailed)
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	   Just (ids, _, bcos) -> do
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		-- update the interactive context
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	        let 
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		    names = map idName ids
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		    -- these names have just been shadowed
		    shadowed = [ n | r <- map nameRdrName names,
				     Just n <- [lookupRdrEnv rn_env r] ]
		    
		    new_rn_env   = extendLocalRdrEnv rn_env names

		    -- remove any shadowed bindings from the type_env
		    filtered_type_env = delListFromNameEnv type_env shadowed

		    new_type_env = extendNameEnvList filtered_type_env 	
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			      		[ (getName id, AnId id)	| id <- ids]

		    new_ic = icontext { ic_rn_env   = new_rn_env, 
			  	  	ic_type_env = new_type_env }

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		-- link it
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		hval <- linkExpr pls bcos
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		-- run it!
		let thing_to_run = unsafeCoerce# hval :: IO [HValue]
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		either_hvals <- sandboxIO thing_to_run
		case either_hvals of
		   Left err
			| err == dEADLOCKED
			-> return ( cmstate{ pcs=new_pcs, ic=new_ic }, 
				    CmRunDeadlocked )
			| otherwise
			-> do hPutStrLn stderr ("unknown failure, code " ++ show err)
			      return ( cmstate{ pcs=new_pcs, ic=new_ic }, CmRunFailed )

		   Right maybe_hvals ->
		     case maybe_hvals of
			Left e -> 
			    return ( cmstate{ pcs=new_pcs, ic=new_ic }, 
				     CmRunException e )
			Right hvals -> do
			     -- Get the newly bound things, and bind them.  
			     -- Don't forget to delete any shadowed bindings from the
			     -- closure_env, lest we end up with a space leak.
			     pls <- delListFromClosureEnv pls shadowed
			     new_pls <- addListToClosureEnv pls (zip names hvals)
	     
			     return (cmstate{ pcs=new_pcs, pls=new_pls, ic=new_ic }, 
				     CmRunOk names)

-- We run the statement in a "sandbox", which amounts to calling into
-- the RTS to request a new main thread.  The main benefit is that we
-- get to detect a deadlock this way, but also there's no danger that
-- exceptions raised by the expression can affect the interpreter.

sandboxIO :: IO a -> IO (Either Int (Either Exception a))
sandboxIO thing = do
  st_thing <- newStablePtr (Exception.try thing)
  alloca $ \ p_st_result -> do
    stat <- rts_evalStableIO st_thing p_st_result
    freeStablePtr st_thing
    if stat == 1
	then do st_result <- peek p_st_result
		result <- deRefStablePtr st_result
		freeStablePtr st_result
		return (Right result)
	else do
		return (Left (fromIntegral stat))

-- ToDo: slurp this in from ghc/includes/RtsAPI.h somehow
dEADLOCKED = 4 :: Int

foreign import "rts_evalStableIO"  {- safe -}
  rts_evalStableIO :: StablePtr (IO a) -> Ptr (StablePtr a) -> IO CInt
  -- more informative than the C type!
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#endif

-----------------------------------------------------------------------------
-- cmTypeOfExpr: returns a string representing the type of an expression

#ifdef GHCI
cmTypeOfExpr :: CmState -> DynFlags -> String -> IO (CmState, Maybe String)
cmTypeOfExpr cmstate dflags expr
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   = do (new_pcs, maybe_stuff) 
	  <- hscStmt dflags hst hit pcs ic expr True{-just an expr-}

	let new_cmstate = cmstate{pcs = new_pcs}

	case maybe_stuff of
	   Nothing -> return (new_cmstate, Nothing)
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	   Just (_, ty, _) -> return (new_cmstate, Just str)
 	     where 
		str = showSDocForUser unqual (ppr tidy_ty)
		unqual  = getUnqual pcs hit ic
		tidy_ty = tidyType emptyTidyEnv ty
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   where
       CmState{ hst=hst, hit=hit, pcs=pcs, ic=ic } = cmstate
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getUnqual pcs hit ic
   = case lookupIfaceByModName hit pit modname of
	Nothing    -> alwaysQualify
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	Just iface -> 
	   case mi_globals iface of
	      Just env -> unQualInScope env
	      Nothing  -> unQualInScope (mkIfaceGlobalRdrEnv (mi_exports iface))
  where
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    pit = pcs_PIT pcs
    modname = moduleName (ic_module ic)
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#endif
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-----------------------------------------------------------------------------
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-- cmTypeOfName: returns a string representing the type of a name.
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#ifdef GHCI
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cmTypeOfName :: CmState -> Name -> IO (Maybe String)
cmTypeOfName CmState{ hit=hit, pcs=pcs, ic=ic } name
 = case lookupNameEnv (ic_type_env ic) name of
	Nothing -> return Nothing
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	Just (AnId id) -> return (Just str)
	   where
	     unqual = getUnqual pcs hit ic
	     ty = tidyType emptyTidyEnv (idType id)
	     str = showSDocForUser unqual (ppr ty)
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	_ -> panic "cmTypeOfName"
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#endif

-----------------------------------------------------------------------------
-- cmCompileExpr: compile an expression and deliver an HValue

#ifdef GHCI
cmCompileExpr :: CmState -> DynFlags -> String -> IO (CmState, Maybe HValue)
cmCompileExpr cmstate dflags expr
   = do 
	let InteractiveContext { 
	       	ic_rn_env = rn_env, 
	       	ic_type_env = type_env,
	       	ic_module   = this_mod } = icontext

        (new_pcs, maybe_stuff) 
	    <- hscStmt dflags hst hit pcs icontext 
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		  ("let __cmCompileExpr = "++expr) False{-stmt-}
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        case maybe_stuff of
	   Nothing -> return (cmstate{ pcs=new_pcs }, Nothing)
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	   Just (ids, _, bcos) -> do
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		-- link it
		hval <- linkExpr pls bcos

		-- run it!
		let thing_to_run = unsafeCoerce# hval :: IO [HValue]
		hvals <- thing_to_run

		case (ids,hvals) of
		  ([id],[hv]) -> return (cmstate{ pcs=new_pcs }, Just hv)
		  _ -> panic "cmCompileExpr"

   where
       CmState{ hst=hst, hit=hit, pcs=pcs, pls=pls, ic=icontext } = cmstate
#endif
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-----------------------------------------------------------------------------
-- Unload the compilation manager's state: everything it knows about the
-- current collection of modules in the Home package.
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cmUnload :: CmState -> DynFlags -> IO CmState
cmUnload state@CmState{ gmode=mode, pls=pls, pcs=pcs } dflags
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 = do -- Throw away the old home dir cache
      emptyHomeDirCache
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      -- Unload everything the linker knows about
      new_pls <- CmLink.unload mode dflags [] pls 

      -- Start with a fresh CmState, but keep the PersistentCompilerState
      new_state <- cmInit mode
      return new_state{ pcs=pcs, pls=new_pls }
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-----------------------------------------------------------------------------
-- Trace dependency graph

-- This is a seperate pass so that the caller can back off and keep
-- the current state if the downsweep fails.

cmDepAnal :: CmState -> DynFlags -> [FilePath] -> IO ModuleGraph
cmDepAnal cmstate dflags rootnames
  = do showPass dflags "Chasing dependencies"
       when (verbosity dflags >= 1 && gmode cmstate == Batch) $
           hPutStrLn stderr (showSDoc (hcat [
	     text progName, text ": chasing modules from: ",
	     hcat (punctuate comma (map text rootnames))]))
       downsweep rootnames (mg cmstate)

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-----------------------------------------------------------------------------
-- The real business of the compilation manager: given a system state and
-- a module name, try and bring the module up to date, probably changing
-- the system state at the same time.
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cmLoadModules :: CmState 
	     -> DynFlags
             -> ModuleGraph
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             -> IO (CmState,		-- new state
		    Bool, 		-- was successful
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		    [String])		-- list of modules loaded
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cmLoadModules cmstate1 dflags mg2unsorted
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   = do -- version 1's are the original, before downsweep
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        let pls1      = pls    cmstate1
        let pcs1      = pcs    cmstate1
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        let hst1      = hst    cmstate1
        let hit1      = hit    cmstate1
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	-- similarly, ui1 is the (complete) set of linkables from
	-- the previous pass, if any.
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        let ui1       = ui     cmstate1
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        let ghci_mode = gmode cmstate1 -- this never changes
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        -- Do the downsweep to reestablish the module graph
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        let verb = verbosity dflags

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	-- Find out if we have a Main module
        let a_root_is_Main 
               = any ((=="Main").moduleNameUserString.name_of_summary) 
                     mg2unsorted
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        let mg2unsorted_names = map name_of_summary mg2unsorted
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        -- reachable_from follows source as well as normal imports
        let reachable_from :: ModuleName -> [ModuleName]
            reachable_from = downwards_closure_of_module mg2unsorted
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        -- should be cycle free; ignores 'import source's
        let mg2 = topological_sort False mg2unsorted
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        -- ... whereas this takes them into account.  Used for
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        -- backing out partially complete cycles following a failed
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        -- upsweep, and for removing from hst/hit all the modules
        -- not in strict downwards closure, during calls to compile.
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        let mg2_with_srcimps = topological_sort True mg2unsorted
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	-- Sort out which linkables we wish to keep in the unlinked image.
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	-- See getValidLinkables below for details.
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	valid_linkables <- getValidLinkables ui1 mg2unsorted_names 
				mg2_with_srcimps
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	-- when (verb >= 2) $
        --    putStrLn (showSDoc (text "Valid linkables:" 
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        -- 			 <+> ppr valid_linkables))
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        -- Figure out a stable set of modules which can be retained
        -- the top level envs, to avoid upsweeping them.  Goes to a
        -- bit of trouble to avoid upsweeping module cycles.
        --
        -- Construct a set S of stable modules like this:
        -- Travel upwards, over the sccified graph.  For each scc
        -- of modules ms, add ms to S only if:
        -- 1.  All home imports of ms are either in ms or S
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        -- 2.  A valid linkable exists for each module in ms

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        stable_mods <- preUpsweep valid_linkables hit1 
		 		  mg2unsorted_names [] mg2_with_srcimps
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        let stable_summaries
               = concatMap (findInSummaries mg2unsorted) stable_mods
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	    stable_linkables
	       = filter (\m -> linkableModName m `elem` stable_mods) 
		    valid_linkables

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        when (verb >= 2) $
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           putStrLn (showSDoc (text "Stable modules:" 
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                               <+> sep (map (text.moduleNameUserString) stable_mods)))
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	-- unload any modules which aren't going to be re-linked this
	-- time around.
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	pls2 <- CmLink.unload ghci_mode dflags stable_linkables pls1
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        -- We could at this point detect cycles which aren't broken by
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        -- a source-import, and complain immediately, but it seems better
        -- to let upsweep_mods do this, so at least some useful work gets
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        -- done before the upsweep is abandoned.
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        let upsweep_these
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               = filter (\scc -> any (`notElem` stable_mods) 
                                     (map name_of_summary (flattenSCC scc)))
                        mg2
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        --hPutStrLn stderr "after tsort:\n"
        --hPutStrLn stderr (showSDoc (vcat (map ppr mg2)))
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        -- Because we don't take into account source imports when doing
        -- the topological sort, there shouldn't be any cycles in mg2.
        -- If there is, we complain and give up -- the user needs to
        -- break the cycle using a boot file.
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        -- Now do the upsweep, calling compile for each module in
        -- turn.  Final result is version 3 of everything.

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        let threaded2 = CmThreaded pcs1 hst1 hit1
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	-- clean up between compilations
	let cleanup = cleanTempFilesExcept verb 
			  (ppFilesFromSummaries (flattenSCCs upsweep_these))

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        (upsweep_complete_success, threaded3, modsUpswept, newLis)
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           <- upsweep_mods ghci_mode dflags valid_linkables reachable_from 
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                           threaded2 cleanup upsweep_these
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        let ui3 = add_to_ui valid_linkables newLis
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        let (CmThreaded pcs3 hst3 hit3) = threaded3

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        -- At this point, modsUpswept and newLis should have the same
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        -- length, so there is one new (or old) linkable for each 
        -- mod which was processed (passed to compile).
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	-- Make modsDone be the summaries for each home module now
	-- available; this should equal the domains of hst3 and hit3.
	-- (NOT STRICTLY TRUE if an interactive session was started
	--  with some object on disk ???)
        -- Get in in a roughly top .. bottom order (hence reverse).

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        let modsDone = reverse modsUpswept ++ stable_summaries
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        -- Try and do linking in some form, depending on whether the
        -- upsweep was completely or only partially successful.

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        if upsweep_complete_success
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         then 
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           -- Easy; just relink it all.
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           do when (verb >= 2) $ 
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		 hPutStrLn stderr "Upsweep completely successful."
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	      -- clean up after ourselves
	      cleanTempFilesExcept verb (ppFilesFromSummaries modsDone)

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	      -- issue a warning for the confusing case where the user said '-o foo'
	      -- but we're not going to do any linking.
	      ofile <- readIORef v_Output_file
	      when (ghci_mode == Batch && isJust ofile && not a_root_is_Main
		     && verb > 0) $
	         hPutStrLn stderr "Warning: output was redirected with -o, but no output will be generated\nbecause there is no Main module."

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	      -- link everything together
              linkresult <- link ghci_mode dflags a_root_is_Main ui3 pls2

	      cmLoadFinish True linkresult 
			hst3 hit3 ui3 modsDone ghci_mode pcs3
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         else 
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           -- Tricky.  We need to back out the effects of compiling any
           -- half-done cycles, both so as to clean up the top level envs
           -- and to avoid telling the interactive linker to link them.
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           do when (verb >= 2) $
		hPutStrLn stderr "Upsweep partially successful."
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              let modsDone_names
                     = map name_of_summary modsDone
              let mods_to_zap_names 
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                     = findPartiallyCompletedCycles modsDone_names 
			  mg2_with_srcimps
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              let mods_to_keep
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                     = filter ((`notElem` mods_to_zap_names).name_of_summary) 
			  modsDone
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              let (hst4, hit4, ui4)
                     = retainInTopLevelEnvs (map name_of_summary mods_to_keep) 
                                            (hst3,hit3,ui3)

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	      -- clean up after ourselves
	      cleanTempFilesExcept verb (ppFilesFromSummaries mods_to_keep)

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	      -- link everything together
              linkresult <- link ghci_mode dflags False ui4 pls2
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	      cmLoadFinish False linkresult 
		    hst4 hit4 ui4 mods_to_keep ghci_mode pcs3


-- Finish up after a cmLoad.
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-- If the link failed, unload everything and return.
cmLoadFinish ok (LinkFailed pls) hst hit ui mods ghci_mode pcs = do
  dflags <- getDynFlags
  new_pls <- CmLink.unload ghci_mode dflags [] pls 
  new_state <- cmInit ghci_mode
  return (new_state{ pcs=pcs, pls=new_pls }, False, [])

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-- Empty the interactive context and set the module context to the topmost
-- newly loaded module, or the Prelude if none were loaded.
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cmLoadFinish ok (LinkOK pls) hst hit ui mods ghci_mode pcs
  = do def_mod <- readIORef defaultCurrentModule
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       let current_mod = case mods of 
				[]    -> def_mod
				(x:_) -> ms_mod x

       	   new_ic = emptyInteractiveContext current_mod

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           new_cmstate = CmState{ hst=hst, hit=hit, ui=ui, mg=mods,
                                  gmode=ghci_mode, pcs=pcs, pls=pls,
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				  ic = new_ic }
           mods_loaded = map (moduleNameUserString.name_of_summary) mods

       return (new_cmstate, ok, mods_loaded)
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-- used to fish out the preprocess output files for the purposes
-- of cleaning up.
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ppFilesFromSummaries summaries
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  = [ fn | Just fn <- map toPpFile summaries ]
  where
   toPpFile sum
     | hspp /= ml_hs_file loc = hspp
     | otherwise              = Nothing
    where
      loc  = ms_location sum
      hspp = ml_hspp_file loc

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-----------------------------------------------------------------------------
-- getValidLinkables

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-- For each module (or SCC of modules), we take:
--
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--	- an on-disk linkable, if this is the first time around and one
--	  is available.
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--
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--	- the old linkable, otherwise (and if one is available).
--
-- and we throw away the linkable if it is older than the source
-- file.  We ignore the on-disk linkables unless all of the dependents
-- of this SCC also have on-disk linkables.
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--
-- If a module has a valid linkable, then it may be STABLE (see below),
-- and it is classified as SOURCE UNCHANGED for the purposes of calling
-- compile.
--
-- ToDo: this pass could be merged with the preUpsweep.

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getValidLinkables
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	:: [Linkable]		-- old linkables
	-> [ModuleName]		-- all home modules
	-> [SCC ModSummary]	-- all modules in the program, dependency order
	-> IO [Linkable]	-- still-valid linkables 

getValidLinkables old_linkables all_home_mods module_graph
  = foldM (getValidLinkablesSCC old_linkables all_home_mods) [] module_graph

getValidLinkablesSCC old_linkables all_home_mods new_linkables scc0
   = let 
	  scc             = flattenSCC scc0
          scc_names       = map name_of_summary scc
	  home_module m   = m `elem` all_home_mods && m `notElem` scc_names
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          scc_allhomeimps = nub (filter home_module (concatMap ms_imps scc))
		-- NOTE: ms_imps, not ms_allimps above.  We don't want to
		-- force a module's SOURCE imports to be already compiled for
		-- its object linkable to be valid.
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	  has_object m = case findModuleLinkable_maybe new_linkables m of
			    Nothing -> False
			    Just l  -> isObjectLinkable l

          objects_allowed = all has_object scc_allhomeimps
     in do

     these_linkables 
	<- foldM (getValidLinkable old_linkables objects_allowed) [] scc

	-- since an scc can contain only all objects or no objects at all,
	-- we have to check whether we got all objects or not, and re-do
	-- the linkable check if not.
     adjusted_linkables 
	<- if objects_allowed && not (all isObjectLinkable these_linkables)
	      then foldM (getValidLinkable old_linkables False) [] scc
	      else return these_linkables

     return (adjusted_linkables ++ new_linkables)


getValidLinkable :: [Linkable] -> Bool -> [Linkable] -> ModSummary 
	-> IO [Linkable]
getValidLinkable old_linkables objects_allowed new_linkables summary 
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  = do let mod_name = name_of_summary summary

       maybe_disk_linkable
          <- if (not objects_allowed)
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		then return Nothing
		else case ml_obj_file (ms_location summary) of
                 	Just obj_fn -> maybe_getFileLinkable mod_name obj_fn
                 	Nothing -> return Nothing

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       let old_linkable = findModuleLinkable_maybe old_linkables mod_name
	   maybe_old_linkable =
	  	case old_linkable of
		    Just l | not (isObjectLinkable l) || stillThere l 
				-> old_linkable
				-- ToDo: emit a warning if not (stillThere l)
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                    other -> Nothing
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	   -- make sure that if we had an old disk linkable around, that it's
	   -- still there on the disk (in case we need to re-link it).
	   stillThere l = 
		case maybe_disk_linkable of
		   Nothing    -> False
		   Just l_disk -> linkableTime l == linkableTime l_disk

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    	   -- we only look for objects on disk the first time around;
    	   -- if the user compiles a module on the side during a GHCi session,
    	   -- it won't be picked up until the next ":load".  This is what the
    	   -- "null old_linkables" test below is.
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           linkable | null old_linkables = maybeToList maybe_disk_linkable
		    | otherwise          = maybeToList maybe_old_linkable

           -- only linkables newer than the source code are valid
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           src_date = ms_hs_date summary
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	   valid_linkable
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	      =  filter (\l -> linkableTime l >= src_date) linkable
		-- why '>=' rather than '>' above?  If the filesystem stores
		-- times to the nearset second, we may occasionally find that
		-- the object & source have the same modification time, 
		-- especially if the source was automatically generated
		-- and compiled.  Using >= is slightly unsafe, but it matches
		-- make's behaviour.
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       return (valid_linkable ++ new_linkables)
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maybe_getFileLinkable :: ModuleName -> FilePath -> IO (Maybe Linkable)
maybe_getFileLinkable mod_name obj_fn
   = do obj_exist <- doesFileExist obj_fn
        if not obj_exist 
         then return Nothing 
         else 
         do let stub_fn = case splitFilename3 obj_fn of
                             (dir, base, ext) -> dir ++ "/" ++ base ++ ".stub_o"
            stub_exist <- doesFileExist stub_fn
            obj_time <- getModificationTime obj_fn
            if stub_exist
             then return (Just (LM obj_time mod_name [DotO obj_fn, DotO stub_fn]))
             else return (Just (LM obj_time mod_name [DotO obj_fn]))

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-----------------------------------------------------------------------------
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-- Do a pre-upsweep without use of "compile", to establish a 
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-- (downward-closed) set of stable modules for which we won't call compile.
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-- a stable module:
--	* has a valid linkable (see getValidLinkables above)
--	* depends only on stable modules
--	* has an interface in the HIT (interactive mode only)

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preUpsweep :: [Linkable]	-- new valid linkables
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	   -> HomeIfaceTable
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           -> [ModuleName]      -- names of all mods encountered in downsweep
           -> [ModuleName]      -- accumulating stable modules
           -> [SCC ModSummary]  -- scc-ified mod graph, including src imps
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           -> IO [ModuleName]	-- stable modules
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preUpsweep valid_lis hit all_home_mods stable []  = return stable
preUpsweep valid_lis hit all_home_mods stable (scc0:sccs)
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   = do let scc = flattenSCC scc0
            scc_allhomeimps :: [ModuleName]
            scc_allhomeimps 
               = nub (filter (`elem` all_home_mods) (concatMap ms_allimps scc))
            all_imports_in_scc_or_stable
               = all in_stable_or_scc scc_allhomeimps
            scc_names
               = map name_of_summary scc
            in_stable_or_scc m
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               = m `elem` scc_names || m `elem` stable
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	    -- now we check for valid linkables: each module in the SCC must 
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	    -- have a valid linkable (see getValidLinkables above).
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	    has_valid_linkable new_summary
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   	      = isJust (findModuleLinkable_maybe valid_lis modname)
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	       where modname = name_of_summary new_summary

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	    has_interface summary = ms_mod summary `elemUFM` hit

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	    scc_is_stable = all_imports_in_scc_or_stable
			  && all has_valid_linkable scc
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			  && all has_interface scc
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        if scc_is_stable
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         then preUpsweep valid_lis hit all_home_mods (scc_names++stable) sccs
         else preUpsweep valid_lis hit all_home_mods stable sccs
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-- Helper for preUpsweep.  Assuming that new_summary's imports are all
-- stable (in the sense of preUpsweep), determine if new_summary is itself
-- stable, and, if so, in batch mode, return its linkable.
findInSummaries :: [ModSummary] -> ModuleName -> [ModSummary]
findInSummaries old_summaries mod_name
   = [s | s <- old_summaries, name_of_summary s == mod_name]

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findModInSummaries :: [ModSummary] -> Module -> Maybe ModSummary
findModInSummaries old_summaries mod
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   = case [s | s <- old_summaries, ms_mod s == mod] of
	 [] -> Nothing
	 (s:_) -> Just s
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-- Return (names of) all those in modsDone who are part of a cycle
-- as defined by theGraph.
findPartiallyCompletedCycles :: [ModuleName] -> [SCC ModSummary] -> [ModuleName]
findPartiallyCompletedCycles modsDone theGraph
   = chew theGraph
     where
        chew [] = []
        chew ((AcyclicSCC v):rest) = chew rest    -- acyclic?  not interesting.
        chew ((CyclicSCC vs):rest)
           = let names_in_this_cycle = nub (map name_of_summary vs)
                 mods_in_this_cycle  
                    = nub ([done | done <- modsDone, 
                                   done `elem` names_in_this_cycle])
                 chewed_rest = chew rest
             in 
             if   not (null mods_in_this_cycle) 
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                  && length mods_in_this_cycle < length names_in_this_cycle
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             then mods_in_this_cycle ++ chewed_rest
             else chewed_rest
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-- Add the given (LM-form) Linkables to the UI, overwriting previous
-- versions if they exist.
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add_to_ui :: UnlinkedImage -> [Linkable] -> UnlinkedImage
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add_to_ui ui lis
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   = filter (not_in lis) ui ++ lis
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     where
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        not_in :: [Linkable] -> Linkable -> Bool
        not_in lis li
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           = all (\l -> linkableModName l /= mod) lis
           where mod = linkableModName li
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data CmThreaded  -- stuff threaded through individual module compilations
   = CmThreaded PersistentCompilerState HomeSymbolTable HomeIfaceTable
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-- Compile multiple modules, stopping as soon as an error appears.
-- There better had not be any cyclic groups here -- we check for them.
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upsweep_mods :: GhciMode
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	     -> DynFlags
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             -> UnlinkedImage         -- valid linkables
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             -> (ModuleName -> [ModuleName])  -- to construct downward closures
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             -> CmThreaded            -- PCS & HST & HIT
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	     -> IO ()		      -- how to clean up unwanted tmp files
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             -> [SCC ModSummary]      -- mods to do (the worklist)
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                                      -- ...... RETURNING ......
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             -> IO (Bool{-complete success?-},
                    CmThreaded,
                    [ModSummary],     -- mods which succeeded
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                    [Linkable])       -- new linkables
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upsweep_mods ghci_mode dflags oldUI reachable_from threaded cleanup
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     []
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   = return (True, threaded, [], [])

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upsweep_mods ghci_mode dflags oldUI reachable_from threaded cleanup
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     ((CyclicSCC ms):_)
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   = do hPutStrLn stderr ("Module imports form a cycle for modules:\n\t" ++
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                          unwords (map (moduleNameUserString.name_of_summary) ms))
        return (False, threaded, [], [])

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upsweep_mods ghci_mode dflags oldUI reachable_from threaded cleanup
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     ((AcyclicSCC mod):mods)
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   = do --case threaded of
        --   CmThreaded pcsz hstz hitz
        --      -> putStrLn ("UPSWEEP_MOD: hit = " ++ show (map (moduleNameUserString.moduleName.mi_module) (eltsUFM hitz)))

        (threaded1, maybe_linkable) 
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           <- upsweep_mod ghci_mode dflags oldUI threaded mod 
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                          (reachable_from (name_of_summary mod))
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	-- remove unwanted tmp files between compilations
	cleanup

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        case maybe_linkable of
           Just linkable 
              -> -- No errors; do the rest
                 do (restOK, threaded2, modOKs, linkables) 
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                       <- upsweep_mods ghci_mode dflags oldUI reachable_from 
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                                       threaded1 cleanup mods
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                    return (restOK, threaded2, mod:modOKs, linkable:linkables)
           Nothing -- we got a compilation error; give up now
              -> return (False, threaded1, [], [])


-- Compile a single module.  Always produce a Linkable for it if 
-- successful.  If no compilation happened, return the old Linkable.
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upsweep_mod :: GhciMode 
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	    -> DynFlags
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            -> UnlinkedImage
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            -> CmThreaded
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            -> ModSummary
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            -> [ModuleName]
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            -> IO (CmThreaded, Maybe Linkable)
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upsweep_mod ghci_mode dflags oldUI threaded1 summary1 reachable_inc_me
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   = do 
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        let mod_name = name_of_summary summary1
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        let (CmThreaded pcs1 hst1 hit1) = threaded1
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        let old_iface = lookupUFM hit1 mod_name
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        let maybe_old_linkable = findModuleLinkable_maybe oldUI mod_name
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            source_unchanged = isJust maybe_old_linkable

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	    reachable_only = filter (/= (name_of_summary summary1)) 
				reachable_inc_me

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	   -- in interactive mode, all home modules below us *must* have an
	   -- interface in the HIT.  We never demand-load home interfaces in
	   -- interactive mode.
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            (hst1_strictDC, hit1_strictDC, [])
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               = ASSERT(ghci_mode == Batch || 
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			all (`elemUFM` hit1) reachable_only)
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		 retainInTopLevelEnvs reachable_only (hst1,hit1,[])
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            old_linkable 
               = unJust "upsweep_mod:old_linkable" maybe_old_linkable

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	    have_object 
	       | Just l <- maybe_old_linkable, isObjectLinkable l = True
	       | otherwise = False

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        compresult <- compile ghci_mode summary1 source_unchanged
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			 have_object old_iface hst1_strictDC hit1_strictDC pcs1
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        case compresult of

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           -- Compilation "succeeded", and may or may not have returned a new
           -- linkable (depending on whether compilation was actually performed
	   -- or not).
           CompOK pcs2 new_details new_iface maybe_new_linkable
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              -> do let hst2      = addToUFM hst1 mod_name new_details
                        hit2      = addToUFM hit1 mod_name new_iface
                        threaded2 = CmThreaded pcs2 hst2 hit2

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                    return (threaded2, if isJust maybe_new_linkable
					  then maybe_new_linkable
					  else Just old_linkable)
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           -- Compilation failed.  compile may still have updated
           -- the PCS, tho.
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           CompErrs pcs2
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	      -> do let threaded2 = CmThreaded pcs2 hst1 hit1
                    return (threaded2, Nothing)
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