[darcs-users] Fwd: darcs patch: fix type witness compile errors specific to ghc 6.8

[David Roundy]

I've applied your patch, by the way...

David

On Tue, Jul 29, 2008 at 12:09 PM, David Roundy <droundy at darcs.net> wrote:
> Argh.  This is a rather annoying regression.  Too bad that SPJ doesn't
> see it as such!  :( But it seems that type inference in monads is
> still safe, so I guess we haven't completely lost the ability to use
> type inference with gadts...
>
> David
>
> On Tue, Jul 29, 2008 at 11:24:28AM -0700, Jason Dagit wrote:
>> I think I didn't use the reply all correctly.
>>
>> Jason
>>
>> ---------- Forwarded message ----------
>> From: Jason Dagit <dagit at codersbase.com>
>> Date: Tue, Jul 29, 2008 at 11:12 AM
>> Subject: Re: [darcs-users] darcs patch: fix type witness compile errors
>> specific to ghc 6.8
>> To: darcs-users at darcs.net
>>
>>
>>
>>
>> On Tue, Jul 29, 2008 at 10:24 AM, David Roundy <droundy at darcs.net> wrote:
>>
>> > Jason,
>> >
>> > I'm willing to accept this patch after you've tried one possible fix
>> > that would avoid this ugliness.  Could you try replacing the $
>> > operator with parentheses instead of defining new functions for these
>> > fixes?  e.g.
>> >
>> > > -mergeUnravelled ws = case reverseFL `fmap` mergeConflictingNons
>> > > -                                           (map sealed2non $ filter
>> > notNullS ws) of
>> > > +mergeUnravelled ws = case mergeUnravelled_private ws of
>> >
>> > would become
>> >
>> > mergeUnravelled ws = case reverseFL `fmap` mergeConflictingNons
>> >                                           (map sealed2non (filter
>> >                                           notNullS ws)) of
>> >
>> > and
>> >
>> > > -          mcn [Sealed p] = case sort_coalesceFL $ effect p of -- this is
>> > just a safety check, and could
>> > > -                           NilFL -> Just p                    -- be
>> > removed when we're sure of the code.
>> > > +          mcn [Sealed p] = case sort_coalesce_effects p of -- this is
>> > just a safety check, and could
>> > > +                           NilFL -> Just p                 -- be
>> >
>> > would become
>> >
>> > mcn [Sealed p] = case sort_coalesceFL (effect p) of -- this is ...
>> >
>> > I'm not sure that this will make things work with 6.8 (since I don't
>> > have ghc 6.8), but it would address the only plausible problem I can
>> > see with the current code (which is considerably prettier than your
>> > fixes).
>> >
>> > If these fixes don't work (as I mentioned above), I'll apply this
>> > patch.
>>
>>
>> Unfortunately for [case ... of ...], in GHC 6.8, and newer, more type
>> annotations are required.  Simon had this to say about it:
>>
>> http://www.haskell.org/pipermail/glasgow-haskell-users/2008-July/015102.html
>>
>> I've read the paper that he linked, which is helpful, but since I'm not much
>> of a type theorist I'm having trouble applying some of it.  What I mean is
>> that, there may be a prettier way to sneak in the rigid types (essentially
>> type annotations), but this was the first one I found that works.  I tried
>> removing ($) but that definitely didn't work out.
>>
>> The good news is that 90% of the type witness code we have doesn't rely
>> heavily on `case` and hasn't required any updates to work on GHC 6.8 and
>> newer.  Also, it seems that the way 6.6 did the type inference was "wrong"
>> and we can count on the way 6.8 does things to be the new status quo.  But,
>> the downside is that (.) seems to be somewhat allergic to case expressions
>> in the presence of GADTs.
>>
>> Adding new functions was just the quickest way to fix this.  Given that
>> sort_coalesce_effects shows up so much in that one module, maybe it's not
>> such a bad thing to define.  Sort of like having, for instance, the zip
>> function that is really just an abbreviation for zipWith (,).
>>
>> I hope that helps,
>> Jason
>
> --
> David Roundy
> Department of Physics
> Oregon State University
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