MIT licensed by David Lazar, Bas van Dijk, David Fox
Maintained by David Fox
This version can be pinned in stack with:process-extras-,1442


Extra functionality for the Process library.

  • Read process input and output as ByteStrings or Text, or write your own ProcessOutput instance.
  • Lazy process input and output (i.e. read output from processes that run forever.)
  • ProcessMaker class for more flexibility in the process creation API.


This project is available on GitHub. You may contribute changes there.

Please report bugs and feature requests using the GitHub issue tracker.


The output type of the raw system process functions is ByteString. Instances of ListLikeProcessIO are provided to read as type String, Text, Lazy Text, ByteString, or Lazy ByteString. Select by casting the result, or by specifying the module containing the specialized function:

> :m +System.Process.ListLike Data.ByteString Data.Text.Lazy
> readCreateProcessWithExitCode (shell "echo 'λ'") mempty :: IO (ExitCode, ByteString, ByteString)
> readCreateProcessWithExitCode (shell "echo 'λ'") mempty :: IO (ExitCode, Text, Text)
> readCreateProcessWithExitCode (shell "echo 'λ'") mempty :: IO (ExitCode, String, String)
> System.Process.Text.readCreateProcessWithExitCode (shell "yes | head -10") mempty

Although the output type can be lazy, normal process functions still need to read until EOF on the process output before returing anything. If you have a process whose output never ends you can use the readCreateProcessLazy function to read it. Functions like readProcess would block waiting for EOF on the process output:

> (Prelude.take 4 <$> readCreateProcessLazy (proc "yes" []) mempty :: IO [Chunk Text]) >>= mapM_ (putStrLn . show)
ProcessHandle <process>
Stdout "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny ..."

The output type can be any instance of ProcessOutput, instances for types (ExitCode, a, a), [Chunk a], and (ExitCode, [Chunk a]) are provided. [Chunk a] can be converted to any other instance of ProcessOutput using collectOutput

> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk ByteString]) >>= mapM_ (putStrLn . show)
Stdout "\US\139\b\NUL\237\136\&7W\NUL\ETX345\183\&403\215\&31Q04267\177\&0\177\212\&33\225\STX\NUL_\169\142\178\ETB\NUL\NUL\NUL"
Stderr "gzip: stdin: file size changed while zipping\n -8.7%\n"
Result ExitSuccess
> (readCreateProcess (shell "uptime") mempty :: IO [Chunk ByteString]) >>= writeOutput
 14:00:34 up 18 days,  7:16,  6 users,  load average: 0.04, 0.10, 0.08
> collectOutput <$> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk ByteString]) :: IO (ExitCode, ByteString, ByteString)
(ExitSuccess,"\US\139\b\NUL\185\137\&7W\NUL\ETX345\183\&427\212\&33W0426731\177\208\&35\225\STX\NUL\237\192\CAN\224\ETB\NUL\NUL\NUL","gzip: stdin: file size changed while zipping\n -8.7%\n")
> collectOutput <$> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk ByteString]) :: IO (ExitCode, ByteString, ByteString)
(ExitSuccess,"\US\139\b\NUL\185\137\&7W\NUL\ETX345\183\&427\212\&33W0426731\177\208\&35\225\STX\NUL\237\192\CAN\224\ETB\NUL\NUL\NUL","gzip: stdin: file size changed while zipping\n -8.7%\n")
> (collectOutput . filter (\x -> case x of Stderr _ -> False; _ -> True)) <$> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk ByteString]) :: IO (ExitCode, ByteString, ByteString)

Some cases that need investigation:

> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk String]) >>= mapM_ (putStrLn . show)
*** Exception: fd:13: hGetContents: invalid argument (invalid byte sequence)
> (readCreateProcess (shell "gzip -v < /proc/uptime") mempty :: IO [Chunk Text]) >>= mapM_ (putStrLn . show)
*** Exception: fd:13: hClose: invalid argument (Bad file descriptor)