[darcs-devel] darcs patch: add context to HashedRepo.copy_repo (and 2 more)

[David Roundy]

On Wed, Mar 12, 2008 at 04:25:29PM -0700, Jason Dagit wrote:
> If it helps to see some of the possibilities and their type errors check
> this page:
> http://hpaste.org/6316

Thanks! This is indeed helpful.  It looks like the primary problem is that
you need to change bits like this:

                    rp [(i,h),(il,hl)] =
                        do Sealed p <- createHashed h (const $ speculate h (reverse allis) >> parse i h)
                           Sealed rest <- rp [(il,hl)]   -- LINE 177
                           return $ seal $ (i `patchInfoAndPatch` p) :<: rest

to bits like this

                    rp [(i,h),(il,hl)] =
                        do Sealed rest <- rp [(il,hl)]   -- LINE 177
                           Sealed p <- createHashed h (const $ speculate h (reverse allis) >> parse i h)
                           return $ seal $ (i `patchInfoAndPatch` p) :<: rest

All I've done is reordered the two computations returning sealed data, so
that the type of the second can be dependent on the type of the first, and
this fixes the compile error.  We can do this a bit more beautifully (in my
opinion) by introducing a higher-order function to do this sort of
combining of two actions that returned sealed results, which can also
introduce an unsafeInterleaveIO, which removes the inelegant change in
order of IO introduced by the above simple fix.

I think it'll be simplest if I record and push a little patch that makes
just this one change.

> I'm annotating it with some of the different things I have tried and their
> type errors.  Maybe this will help step you through the difficulties I've
> had with getting read_repo_private to work.

Alas, this is some tricky stuff.  I wish we could make the error messages
more obvious!
-- 
David Roundy
Department of Physics
Oregon State University