Help the user when she tries to do :history without :trace

[ghc-hetmet.git] / compiler / ghci / InteractiveUI.hs

diff --git a/compiler/ghci/InteractiveUI.hs b/compiler/ghci/InteractiveUI.hs
index 3ae37f5..4c9bac0 100644 (file)
--- a/compiler/ghci/InteractiveUI.hs
+++ b/compiler/ghci/InteractiveUI.hs
@@ -846,6 +846,12 @@ addModule files = do
   afterLoad ok session False prev_context
 
 changeDirectory :: String -> GHCi ()
+changeDirectory "" = do
+  -- :cd on its own changes to the user's home directory
+  either_dir <- io (IO.try getHomeDirectory)
+  case either_dir of
+     Left _e -> return ()
+     Right dir -> changeDirectory dir
 changeDirectory dir = do
   session <- getSession
   graph <- io (GHC.getModuleGraph session)
@@ -1906,15 +1912,19 @@ historyCmd arg
       (r:_) -> do
         let hist = GHC.resumeHistory r
             (took,rest) = splitAt num hist
-        spans <- mapM (io . GHC.getHistorySpan s) took
-        let nums  = map (printf "-%-3d:") [(1::Int)..]
-        let names = map GHC.historyEnclosingDecl took
-        printForUser (vcat(zipWith3 
-                             (\x y z -> x <+> y <+> z) 
-                             (map text nums) 
-                             (map (bold . ppr) names)
-                             (map (parens . ppr) spans)))
-        io $ putStrLn $ if null rest then "<end of history>" else "..."
+        case hist of
+          [] -> io $ putStrLn $ 
+                   "Empty history. Perhaps you forgot to use :trace?"
+          _  -> do
+                 spans <- mapM (io . GHC.getHistorySpan s) took
+                 let nums  = map (printf "-%-3d:") [(1::Int)..]
+                     names = map GHC.historyEnclosingDecl took
+                 printForUser (vcat(zipWith3 
+                                 (\x y z -> x <+> y <+> z) 
+                                 (map text nums) 
+                                 (map (bold . ppr) names)
+                                 (map (parens . ppr) spans)))
+                 io $ putStrLn $ if null rest then "<end of history>" else "..."
 
 bold :: SDoc -> SDoc
 bold c | do_bold   = text start_bold <> c <> text end_bold