diff --git a/machines-process.cabal b/machines-process.cabal
--- a/machines-process.cabal
+++ b/machines-process.cabal
@@ -1,5 +1,5 @@
 name:                machines-process
-version:             0.2.0.4
+version:             0.2.0.6
 synopsis:            Process (system) utilities for the machines library
 homepage:            http://github.com/aloiscochard/machines-process
 license:             Apache-2.0
@@ -19,6 +19,6 @@
   build-depends:       
       base                >= 4.6      && < 5
     , chunked-data        >= 0.1      && < 0.3
-    , machines            >= 0.2      && < 0.6
+    , machines            >= 0.2      && < 0.7
     , machines-io         >= 0.2      && < 0.3
     , process             >= 1.1      && < 1.3
diff --git a/src/System/Process/Machine.hs b/src/System/Process/Machine.hs
--- a/src/System/Process/Machine.hs
+++ b/src/System/Process/Machine.hs
@@ -17,6 +17,12 @@
 mStdOut mp (_, Just stdOut, _)  = runT $ mp <~ stdOut
 mStdOut _  _                    = return []
 
+{--
+mStdOut_ :: ProcessT IO a b -> ProcessMachines a a0 k0 -> IO ()
+mStdOut mp (_, Just stdOut, _)  = runT_ $ mp <~ stdOut
+mStdOut _  _                    = return ()
+--}
+
 mStdErr :: ProcessT IO a b -> ProcessMachines a a0 k0 -> IO [b]
 mStdErr mp (_, _, Just stdErr)  = runT $ mp <~ stdErr
 mStdErr _  _                    = return []