[Haskell-cafe] containers-deepseq?

[Joachim Breitner]

Hi,

Am Dienstag, den 29.11.2011, 10:36 +0200 schrieb Michael Snoyman:
> On Tue, Nov 29, 2011 at 10:29 AM, Joachim Breitner
> <mail at joachim-breitner.de> wrote:
> > Am Dienstag, den 29.11.2011, 07:43 +0200 schrieb Michael Snoyman:
> >> Since the release of deepseq 1.2, we've had a bit of a problem: when
> >> using the newest versions of packages on Hackage, there is no NFData
> >> instance available for the containers types. When GHC 7.4 comes out
> >> with its newer version of containers, this will be addressed, but
> >> we'll still have problems for users of older GHC releases.
> >>
> >> I'd like to suggest a solution: a new package called
> >> containers-deepseq. Using conditional compilation, it will provide an
> >> orphans NFData instance for all containers types when deepseq >= 1.2
> >> and containers < 0.5. Packages (e.g., aeson) would then depend on
> >> containers-deepseq and simply import the module whenever they rely on
> >> the NFData instances. When GHC 7.4 comes out, the package will
> >> essentially be a no-op.
> >>
> >> Does this make sense?
> >
> > from a distro point of view: Please void this if possible; every
> > additional package causes us work. (Although we’d likely just not
> > upgrade containers before 7.4.1 and patch out the dependency in the
> > cabal file when we switch to 7.4.1.)
> >
> > Why can’t you put the instances in containers, guarded by some #ifs? I
> > don’t see the point of a separate package for these 15 lines of code.
> 
> Because containers cannot be updated: you're stuck with whatever
> version comes with GHC.

ah, I remember. Trying the same trick in deepseq is also not nice,
because it would require a cabal-conditional dependency on containers,
which would surely cause other problems.

How about depending on 
  (deepseq < 1.2 && containers < 0.5) || (deepseq >= 1.2 && containers >= 0.5)
in a package that uses the NFData instances? Then cabal would figure out
the right version of deepseq to use, based on the version of containers
available (or at least I hope so).

Gruß,
Joachim

-- 
Joachim "nomeata" Breitner
  mail at joachim-breitner.de  |  nomeata at debian.org  |  GPG: 0x4743206C
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