diff --git a/Changelog.md b/Changelog.md
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,8 @@
+### 0.1.2.1
+
+- Added Cabal flag *debug* (off by default) for printing how vim runs to
+  *stderr*.
+
 ### 0.1.2.0
 
 - Better treatment of handles of temporary files.
diff --git a/pandoc-vimhl.cabal b/pandoc-vimhl.cabal
--- a/pandoc-vimhl.cabal
+++ b/pandoc-vimhl.cabal
@@ -1,5 +1,5 @@
 name:                pandoc-vimhl
-version:             0.1.2.0
+version:             0.1.2.1
 synopsis:            Pandoc filter for native Vim code highlighting
 description:         Pandoc filter for native Vim code highlighting in HTML and
         PDF documents. Requires Vim and plugin
@@ -15,14 +15,24 @@
 build-type:          Simple
 cabal-version:       >= 1.8
 
+flag debug
+  description:       Enable debug support (print how vim runs to stderr)
+  default:           False
+  manual:            True
+
 executable vimhl
-  build-depends:       base >= 4.7 && < 5
-                     , pandoc-types >= 1.12
-                     , directory
-                     , filepath
-                     , process
-                     , temporary >= 1.1
-                     , regex-compat
-                     , cond >= 0.2
-  main-is:             vimhl.hs
+  build-depends:     base >= 4.7 && < 5
+                   , pandoc-types >= 1.12
+                   , directory
+                   , filepath
+                   , process
+                   , temporary >= 1.1
+                   , regex-compat
+                   , cond >= 0.2
+  extensions:        CPP
+
+  if flag(debug)
+    cpp-options:    -DDEBUG
+
+  main-is:           vimhl.hs
 
diff --git a/vimhl.hs b/vimhl.hs
--- a/vimhl.hs
+++ b/vimhl.hs
@@ -1,5 +1,7 @@
 #!/usr/bin/env runhaskell
 
+{-# LANGUAGE CPP #-}
+
 -- vimhl.hs
 import Text.Pandoc.JSON
 import Text.Regex (mkRegex, splitRegex, matchRegexAll)
@@ -53,6 +55,16 @@
             hPutStr hsrc contents >> hFlush hsrc
             bracket (emptySystemTempFile "_vimhl_dst.") removeFile $
                 \dst -> do
+                    let vimcmd =
+                            unwords
+                                ["vim -Nen", cmds, vimrccmd, colorscheme
+                                ,"-c 'set ft=" ++ ft, "|"
+                                ,vimhlcmd ++ "' -c 'w!", dst ++ "' -c 'qa!'"
+                                ,src
+                                ]
+#ifdef DEBUG
+                    hPutStr stderr $ vimcmd ++ " ... "
+#endif
                     {- vim must think that it was launched from a terminal,
                      - otherwise it won't load its usual environment and the
                      - syntax engine! Using WriteMode for stdin prevents vim
@@ -62,15 +74,12 @@
                      - mode of the handle). -}
                     hin <- openFile "/dev/tty" WriteMode
                     hout <- openFile "/dev/null" WriteMode
-                    (_, _, _, handle) <- createProcess
-                        (shell $ unwords
-                            ["vim -Nen", cmds, vimrccmd, colorscheme
-                            ,"-c 'set ft=" ++ ft, "|", vimhlcmd ++ "' -c 'w!"
-                            ,dst ++ "' -c 'qa!'", src
-                            ]
-                        ) {std_in = UseHandle hin, std_out = UseHandle hout}
+                    (_, _, _, handle) <- createProcess (shell vimcmd)
+                        {std_in = UseHandle hin, std_out = UseHandle hout}
                     r <- waitForProcess handle
-                    unless (r == ExitSuccess) $ exitWith r
+#ifdef DEBUG
+                    hPutStrLn stderr "done"
+#endif
                     readFile dst
         return $ RawBlock fm block
     where namevals' = map (first $ map toLower) namevals