I think the following error message is a bit misleading:

$ darcs pull me@typo:valid_path

darcs failed:  Not a repository: me@typo:valid_path ((scp) failed to fetch:
me@typo:valid_path/_darcs/inventory)


We should change the error message to be more precise in this case.  Everything
was good except the host name, so it would have been nice to see a "host not
found" or similar.

The error message with get seems to be similarly imprecise:
$ darcs get http://blah

darcs failed:  Not a repository: http://blah (failed to fetch:
http://blah/_darcs/inventory ExitFailure 1)

I agree that a more specific error out to be available and we ought be able to
bubble that up to the user. For example, if I 'scp' to a typo'ed domain, I get
this back:

ssh: typoed-domain.com: Name or service not known

It seems to me that we ought to be able to capture something like that and pass
it back through to the user.

I took a look at this issue. There are a number of things to consider:

1) scp returns a -1 exit code when host is not found. Since -1 is
returned for general failures, the only way to know what really happened
is inspecting the output.

2) In the file Darcs/Repository/Format.hs:113, all a catchall exception
guard is used and the result is set to an empty Bytestring if failed.
Then a generic error message is given.

To solve (1) we either use an ssh library for Haskell or we read the
output of the ssh command. If you don't mind the extra dependency,
http://hackage.haskell.org/package/knob, seems worth trying to dump the
output directly to memory. Otherwise a temporary file handle has to be used.
To solve (2) We can throw an ExecException with the error message from
scp on failure and catch that exception in Format.hs for a better
message. Or change the function fetchFilePS to return an either type
which is a more Haskell way of doing it.

I am happy to work on this issue by reading the output from ssh and
changing the type of fetchFilePS but since I have not contributed to
Darcs, would like to know if my approach is reasonable.

Thanks a lot for volunteering - anything you can do would be very 
welcome. I think your approach sounds just right.

I'd go with the Either type rather than an exception, as you say 
it's the more Haskell way.

Is it really necessary to use a separate package to hook up the 
output from one command - can't we instead get the Handles from 
running the process with e.g. runInteractiveProcess? It's not 
something I'm particularly familiar with though so I may well be 
missing something.

If it is necessary, it's fine to use knob. There's also bytestring-
handle which I wrote a while ago because I thought it would be 
needed to solve some encoding issues in darcs, but in the end it 
didn't work out, so the package isn't necessarily well tested etc.

Thanks for your feedback. I will work on it. I will try asking what is
the best way to get the output of a process w/o using temporary files or
extra packages. I hope I can get a good solution.

The canonical package that everyone uses for this kind of stuff is
http://hackage.haskell.org/package/process

Resolved by Patch1246