Proposal: overhaul System.Process

David Roundy droundy at
Fri May 16 11:10:43 EDT 2008

On Fri, May 16, 2008 at 02:34:48PM +0100, Neil Mitchell wrote:
> >  - made it work on Windows, and the patch passes GHC's validate
> >    on both Windows and Linux.
> >
> > Reminder: the deadline for discussion is 20 May (5 days).
> Fully support. I think it could benefit from a slight clarification to
> the documentation in createProcess, to help pattern-match safety
> checking.
> (aStdin,aStdout,aStderr,d) <- createProcess cp
> isJust aStdin == (std_in == CreatePipe)
> (and ditto for the out and err)

If only we had type families already, then we could define

-- These data constructors are exported
data Inherit = Inherit
data CreatePipe = CreatePipe
data UseHandle = UseHandle Handle
data NewHandle = NewHandle Handle

-- This class is not exported! (so CreateProcess needn't worry about new
--                              instances sowing up)
class StdStream s where
  type Out s
  isInherit :: s -> Bool
  isCreatePipe :: s -> Bool
  isHandle :: s -> Maybe Handle

instance StdStream Inherit where
  type Out Inherit = ()

instance StdStream IsHandle where
  type Out IsHandle = ()

instance StdStream CreateProcess where
  type Out Inherit = NewHandle

data CreateProcess sin sout serr = CreateProcess {
  cmdspec :: CmdSpec
  cwd :: (Maybe FilePath)
  env :: (Maybe [(String, String)])
  std_in :: sin
  std_out :: sout
  std_err :: serr
  close_fds :: Bool

createProcess :: (StdStream sin, StdStream sout, StdStream serr) =>
                 CreateProcess sin sout serr
              -> IO (Out sin, Out sout, Out serr, ProcessHandle)

Then we could have a static guarantee that we only try to peek at
actually-created pipes.  I suppose this is a bit heavy infrastructure just
to avoid runtime checks for "Just", but in a few years (say, post
Haskell'...) it'd be nice to have safer instances like this.


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