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

On Sun, 2009-11-29 at 09:55 -0800, Alexander Dunlap wrote:

On Sun, Nov 29, 2009 at 5:37 AM, Duncan Coutts 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.

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. Duncan