Improve the "could not find module" error message

In particular, if we're searching for the profiling version of a
module in another package, then suggest that perhaps it might not have
been installed.

compiler/iface/LoadIface.lhs

@@ -48,7 +48,7 @@ import SrcLoc		( importedSrcLoc )
 import Maybes		( MaybeErr(..) )
 import ErrUtils         ( Message )
 import Finder		( findImportedModule, findExactModule,  
-			  FindResult(..), cantFindError )
+			  FindResult(..), cannotFindInterface )
 import UniqFM
 import Outputable
 import BinIface		( readBinIface )
@@ -81,14 +81,11 @@ loadSrcInterface doc mod want_boot  = do
     Found _ mod -> do
       mb_iface <- initIfaceTcRn $ loadInterface doc mod (ImportByUser want_boot)
       case mb_iface of
-	Failed err      -> failWithTc (elaborate err)
+	Failed err      -> failWithTc err
 	Succeeded iface -> return iface
     err ->
         let dflags = hsc_dflags hsc_env in
-	failWithTc (elaborate (cantFindError dflags mod err))
-  where
-    elaborate err = hang (ptext SLIT("Failed to load interface for") <+> 
-			  quotes (ppr mod) <> colon) 4 err
+	failWithTc (cannotFindInterface dflags mod err)
 
 -- | Load interfaces for a collection of orphan modules.
 loadOrphanModules :: [Module] -> TcM ()
@@ -420,7 +417,8 @@ findAndReadIface doc_str mod hi_boot_file
 	      Failed err -> do
 		{ traceIf (ptext SLIT("...not found"))
 		; dflags <- getDOpts
-		; returnM (Failed (cantFindError dflags (moduleName mod) err)) } ;
+		; returnM (Failed (cannotFindInterface dflags 
+					(moduleName mod) err)) } ;
 
 	      Succeeded file_path -> do 
 

compiler/main/Finder.lhs

@@ -19,7 +19,8 @@ module Finder (
     findObjectLinkableMaybe,
     findObjectLinkable,
 
-    cantFindError,
+    cannotFindModule,
+    cannotFindInterface,
   ) where
 
 #include "HsVersions.h"
@@ -144,10 +145,10 @@ findExactModule hsc_env mod =
 this `orIfNotFound` or_this = do
   res <- this
   case res of
-    NotFound here -> do
+    NotFound here _ -> do
 	res2 <- or_this
 	case res2 of
-	   NotFound or_here -> return (NotFound (here ++ or_here))
+	   NotFound or_here pkg -> return (NotFound (here ++ or_here) pkg)
 	   _other -> return res2
     _other -> return res
 
@@ -168,7 +169,7 @@ homeSearchCache hsc_env mod_name do_this = do
 findExposedPackageModule :: HscEnv -> ModuleName -> IO FindResult
 findExposedPackageModule hsc_env mod_name
         -- not found in any package:
-  | null found = return (NotFound [])
+  | null found = return (NotFound [] Nothing)
         -- found in just one exposed package:
   | [(pkg_conf, _)] <- found_exposed
         = let pkgid = mkPackageId (package pkg_conf) in      
@@ -329,7 +330,7 @@ searchPathExts paths mod exts
 	              file = base `joinFileExt` ext
 		]
 
-    search [] = return (NotFound (map fst to_search))
+    search [] = return (NotFound (map fst to_search) (Just (modulePackageId mod)))
     search ((file, mk_result) : rest) = do
       b <- doesFileExist file
       if b 
@@ -505,14 +506,19 @@ dots_to_slashes = map (\c -> if c == '.' then '/' else c)
 -- -----------------------------------------------------------------------------
 -- Error messages
 
-cantFindError :: DynFlags -> ModuleName -> FindResult -> SDoc
-cantFindError dflags mod_name (FoundMultiple pkgs)
-  = hang (ptext SLIT("Cannot import") <+> quotes (ppr mod_name) <> colon) 2 (
+cannotFindModule :: DynFlags -> ModuleName -> FindResult -> SDoc
+cannotFindModule = cantFindErr SLIT("Could not find module")
+
+cannotFindInterface  :: DynFlags -> ModuleName -> FindResult -> SDoc
+cannotFindInterface = cantFindErr SLIT("Failed to load interface for")
+
+cantFindErr cannot_find dflags mod_name (FoundMultiple pkgs)
+  = hang (ptext cannot_find <+> quotes (ppr mod_name) <> colon) 2 (
        sep [ptext SLIT("it was found in multiple packages:"),
 		hsep (map (text.packageIdString) pkgs)]
     )
-cantFindError dflags mod_name find_result
-  = hang (ptext SLIT("Could not find module") <+> quotes (ppr mod_name) <> colon)
+cantFindErr cannot_find dflags mod_name find_result
+  = hang (ptext cannot_find <+> quotes (ppr mod_name) <> colon)
        2 more_info
   where
     more_info
@@ -529,17 +535,31 @@ cantFindError dflags mod_name find_result
 		-> ptext SLIT("no package matching") <+> ppr pkg <+>
 		   ptext SLIT("was found")
 
-	    NotFound files
+	    NotFound files mb_pkg
 		| null files
 		-> ptext SLIT("it is not a module in the current program, or in any known package.")
-		| verbosity dflags < 3 
-		-> ptext SLIT("use -v to see a list of the files searched for")
-		| otherwise 
-		-> hang (ptext SLIT("locations searched:")) 
-		      2 (vcat (map text files))
+		| Just pkg <- mb_pkg, pkg /= thisPackage dflags, build_tag /= ""
+		-> let 
+		     build = if build_tag == "p" then "profiling" 
+						 else "\"" ++ build_tag ++ "\""
+		   in
+		   ptext SLIT("Perhaps you haven't installed the ") <> text build <>
+		   ptext SLIT(" libraries for package ") <> ppr pkg <> char '?' $$
+		   not_found files
+
+		| otherwise
+		-> not_found files
 
 	    NotFoundInPackage pkg
 		-> ptext SLIT("it is not in package") <+> ppr pkg
 
 	    _ -> panic "cantFindErr"
+
+    build_tag = buildTag dflags
+
+    not_found files
+	| verbosity dflags < 3
+	= ptext SLIT("Use -v to see a list of the files searched for.")
+	| otherwise 
+	= hang (ptext SLIT("locations searched:")) 2 (vcat (map text files))
 \end{code}

compiler/main/GHC.hs

@@ -1631,7 +1631,7 @@ preprocessFile dflags src_fn mb_phase (Just (buf, time))
 noModError :: DynFlags -> SrcSpan -> ModuleName -> FindResult -> IO ab
 -- ToDo: we don't have a proper line number for this error
 noModError dflags loc wanted_mod err
-  = throwDyn $ mkPlainErrMsg loc $ cantFindError dflags wanted_mod err
+  = throwDyn $ mkPlainErrMsg loc $ cannotFindModule dflags wanted_mod err
 				
 noHsFileErr loc path
   = throwDyn $ mkPlainErrMsg loc $ text "Can't find" <+> text path
@@ -1850,7 +1850,7 @@ findModule' hsc_env mod_name maybe_pkg =
 	  case res of
 	    Found _ m | modulePackageId m /= this_pkg -> return m
                         -- not allowed to be a home module
-	    err -> let msg = cantFindError dflags mod_name err in
+	    err -> let msg = cannotFindModule dflags mod_name err in
 		   throwDyn (CmdLineError (showSDoc msg))
 
 #ifdef GHCI

compiler/main/HscTypes.lhs

@@ -323,8 +323,8 @@ data FindResult
   | ModuleHidden  PackageId
 	-- for an explicit source import: the package containing the module is
 	-- exposed, but the module itself is hidden.
-  | NotFound [FilePath]
-	-- the module was not found, the specified places were searched.
+  | NotFound [FilePath] (Maybe PackageId)
+	-- the module was not found, the specified places were searched
   | NotFoundInPackage PackageId
 	-- the module was not found in this package