[darcs-users] darcs patch: Resolve issue1292: re-encode line input from the Haskeline backend.

[Judah Jacobson]

On Thu, Jan 22, 2009 at 1:59 AM, Eric Kow <kowey at darcs.net> wrote:
> By the way, I now get this complaint:
>
> src/Darcs/Utils.hs:49:7:
>    Could not find module `System.Console.Haskeline.Encoding':
>          Use -v to see a list of the files searched for.
>
> which I resolved with cabal update and cabal upgrade haskeline
>
> Could you update the configure test accordingly?  Thanks!

I ran into a problem trying to do this.  The only API change between
0.5 and 0.6 was the addition of System.Console.Haskeline.Encoding, so
I tried:

GHC_CHECK_MODULE(System.Console.Haskeline.Encoding( encode ),
                 haskeline,
                 encode ,,
                 AC_MSG_ERROR(Cannot find Haskeline; try installing
the Haskeline package >=0.6.0.))

but when autoconf compiles that it gets:
conftest.hs:2:12:
    Ambiguous type variable `m' in the constraint:
      `mtl-1.1.0.2:Control.Monad.Trans.MonadIO m'
        arising from a use of `encode' at conftest.hs:2:12-17

I.e., it runs into the monomorphism restriction since encode has an
overloaded type.

The solution would seem to be to specify the type:
encode :: MonadIO m => String -> InputT m ByteString
but specifying that type requires importing two other modules
(Control.Monad.Trans and System.Console.Haskeline).  Is that possible
with the current autoconf macros?

Thanks,
-Judah