Thanks for the very nice explanation! Does that mean that we have the
invariant that in
PIAP i (Hopefully Actually p), we have p = (Named i _ _), the two i
being the same?
Florent
Le 30 sept. 08 à 20:18, Jason Dagit a écrit :
On Tue, Sep 30, 2008 at 8:08 AM, Florent Becker <[EMAIL PROTECTED]
> wrote:
Hi list,
as a newbie darcs hacker, i have a ton of naïve questions. If you
don't
mind me asking them, i'll try to compile your answers into darcs-doc
strings in the code for the next generation of darcs hackers.
The first one is: what problem does the 'PatchInfoAnd' type solve? Why
don't we call 'Darcs.Patch.Core.patch2patchinfo' everywhere, and
what is
the Hopefully for?
The problem solved by PatchInfoAnd is the one where you know the
info for a patch and you may be able to fetch that patch but you're
not certain. This comes up in the following scenario: Imagine
reading a patch info from an inventory and then still being faced
with the problem of finding the patch either in the local
repository, the cache, or needing to fetch the patch lazily. I'm
not sure if this code is used with partial repos, but it may help
there as well.
It works like this:
data Named p C(x y) where
NamedP :: !PatchInfo -> ![PatchInfo] -> !(p C(x y)) -> Named p C(x
y)
data Hopefully a C(x y) = Hopefully (SimpleHopefully a C(x y)) |
Hashed String (SimpleHopefully a C(x y))
data SimpleHopefully a C(x y) = Actually (a C(x y)) | Unavailable
String
data PatchInfoAnd p C(a b) = PIAP !PatchInfo (Hopefully (Named p)
C(a b))
patch2patchinfo :: Named p C(x y) -> PatchInfo
patch2patchinfo (NamedP i _ _) = i
Clearly, we see that given any Named patch we can use
patch2patchinfo to get the patch info.
As in the thread Eric pointed you to, you can see that
SimpleHopefully really a new name for 'Either String (a C(x y))',
but then we added one more layer on top of that, called Hopefully,
and we can see that Hopefully is equvalent to:
Either (Either String (a C(x y))) (String, (Either String (a C(x y))))
I think I have that right, the right side of Hopefully is a bit
tricky. Clearly it's better to give our own data type. Nested
Eithers are just ugly.
Back to the point. SimpleHopefully encodes the fetchable status of
the patch. We either 'Actually' have the patch or it's
'Unavailable'. I'm not sure what we store in the unavailable
String. We might store the location of the patch or we might store
an error string. I'd have to dig deeper to know. Maybe you could do
that or someone else could answer the question.
Now that we store whether the patch is available we have another
layer in Hopefully. In Hopefully we either have just a plain old
darcs patch or we have one of the fancy newer patches that comes
with a hash, eg., Hashed String (SimpleHopefully a C(x y)).
Then we have Named, which just stores a patch and it's info (which
is the name). With a Named there is no uncertainty about whether
you have the patch, it's here and ready to be examined. Whereas
you'll notice lots of functions in the Hopefully module that are for
looking up patches and dealing with the error cases.
I hope this helps,
Jason
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