The process-extras package

This is a package candidate release! Here you can preview how this package release will appear once published to the main package index (which can be accomplished via the 'maintain' link below). Please note that once a package has been published to the main package index it cannot be undone! Please consult the package uploading documentation for more information.


Extends Read process input and output as ByteStrings or Text, or write your own ProcessOutput instance. Lazy process input and output. ProcessMaker class for more flexibility in the process creation API.

[Skip to ReadMe]


Versions0.1.0, 0.1.1, 0.1.2, 0.1.3, 0.2.0, 0.3.0,, 0.3.1,,,,,,,,, 0.3.4, 0.4, 0.4.1,,,,, 0.7, 0.7.1
Change logchangelog
Dependenciesbase (>=4.5 && <5), bytestring, deepseq (>=1.1), generic-deriving (>=1.10), ListLike (>=4), process, text [details]
AuthorDavid Lazar, Bas van Dijk, David Fox
MaintainerDavid Fox <>
Home page
Source repositoryhead: git clone
UploadedSun May 15 16:24:34 UTC 2016 by DavidFox




Maintainers' corner

For package maintainers and hackage trustees

Readme for process-extras-


Extra functionality for the Process library.


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)