[Haskell-cafe] instance Binary UTCTime (Was: Oprhan instances)

Alexander Dunlap alexander.dunlap at gmail.com
Sun Nov 29 22:38:23 EST 2009

On Sun, Nov 29, 2009 at 4:41 PM, Duncan Coutts
<duncan.coutts at googlemail.com> wrote:
> On Sun, 2009-11-29 at 09:55 -0800, Alexander Dunlap wrote:
>> On Sun, Nov 29, 2009 at 5:37 AM, Duncan Coutts
>> <duncan.coutts at googlemail.com> wrote:
>> > On Thu, 2009-11-26 at 16:40 -0500, David Menendez wrote:
>> >
>> >> The problem with this solution is that it doesn't scale. If we have M
>> >> packages providing types and N packages providing classes, then we
>> >> need M*N additional packages for orphans.
>> >>
>> >> The best long-term solution is probably extending Cabal to handle this
>> >> more transparently, perhaps by allowing packages to depend on flagged
>> >> versions of other packages (e.g., foomonad >= 4.0 && < 4.1 && HAS_MTL)
>> >
>> > Not going to happen. Such packages could not be translated into binary
>> > distro packages.
>> >
>> > Duncan
>> >
>> Wouldn't the distro just choose one set of flags for each package and
>> then other packages would either be satisfied or not satisfied based
>> on which flags had been chosen?
> Here's the system I assumed you were talking about. You can tell me if I
> misunderstood.
> Instead of having N * M packages, you have a package that provides
> "optional" instances. For example package A defines a class and
> optionally provides instances for types defined in B. If you select to
> have it depend on B then the instances are provided, otherwise not.
> In a source based system this seems to work ok, you would provide
> optional instances for all the packages you already happen to have
> installed. Though if later you install another package that could have
> had optional instances provided then you have to go recompiling things.
> It's slightly worse for binary packages because the distro has to decide
> up front if they're going to provide the optional instances or not.
> Since someone might need them then you end up picking the maximal set of
> optional dependencies and you end up pulling in all sorts of apparently
> unrelated packages.
> Then the other bit you suggested foomonad >= 4.0 && < 4.1 && HAS_MTL
> would be needed to be able to express that you want a package that has
> been built with a particular optional instance provided. This is the bit
> that cannot be translated into packages in most distros. Yes you could
> pick the flags up front, but you have to pick a single assignment that
> satisfies everyone.

Well, that happens anyway with most packages since distros have to
choose one set of flags that works. The proposal I was commenting on
would just allow packages to depend on flags of other packages and so
be explicit about this.

>> It seems to me that distros could even offer multiple options for the
>> same package with different flags set.
> Most distros cannot handle installing multiple instances of the same
> version of a package.

Well, what I've seen is having different packages, i.e.
foo-quickcheck, foo-no-quickcheck as separate packages.

(Note that I can't take credit for suggesting the idea, I was just
asking you about your objection.)


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