POpen provides a convenient way of sending string input to a subprocess and reading output from it lazily.

popen :: FilePath			-- Command
      -> [String]			-- Arguments
      -> Maybe String			-- Input
      -> IO (String, String, ProcessID)	-- (stdout, stderr, pid)

popen cmd args inp executes cmd with args in a forked process. If inp is Just str then str in sent in a pipe to the standard input of the process. The output and error streams from the process are returned, together with the process id.

popenEnvDir :: FilePath				-- Command
	    -> [String]				-- Arguments
	    -> Maybe String			-- Input
	    -> Maybe [(String, String)]		-- Environment
	    -> Maybe FilePath 			-- Working directory    
	    -> IO (String, String, ProcessID)	-- (stdout, stderr, pid)

popenEnvDir cmd args inp env dir like popen executes cmd with args in a forked process. If inp is Just str then str in sent in a pipe to the standard input of the process. If env is Just pairs, the command in executed in the environment specified by pairs, instead of the current one. If dir is Just d the command is executed in directory d instead of the current directory. The output and error streams from the process are returned, together with the process id.