* Andreas Enge <andreas@enge.fr> [2015-12-27 10:48:32+0100]
> On Sun, Dec 27, 2015 at 12:20:27PM +0300, Dmitry Bogatov wrote:
> > Currently, I am at master branch. I want install parallel-20151122.
> > But it is gone since 0877e. I propose to keep *all* versions,
> > but just 'parallel' refer to latest.
> 
> This would be a nightmare to maintain. And what do you do about security
> updates? If libfoo-1.1.7 fixes a critical security bug, who would backport
> this to libfoo-1.0.x and libfoo-1.1.0 to libfoo-1.1.6?

Drop old versions, or provide a warning to user in such emergency case.
After all, user have right to believe, that this particular bug is
not relevant to him

> Then there is the combinatorial explosion. If you have 20 libraries in
> 10 versions each that are needed to build a derived binary, then there
> will be 10^20 possible combinations. Which of them would you like to
> support?

Build binaries with latest versions of libraries and compilers.

> Our general policy is to keep only the latest version, except for special
> cases where people see a point in keeping older versions (script languages,
> libraries like qt with two major versions supported in parallel, and so on).

> What is your use case? If you want reproducibility, it could make sense
> to simply stick to a given git commit. If you just need a particular older

My use case is that I want to be able to install every version of
any haskell library and every version of ghc for testing purposes.

For example, I write code, that uses ghc-7.10 with lens-4.13. Will it
work with ghc-7.6(Debian stable) and lens-4.1.2.1? This versions are
important to me, but some other person may have another reference.

Yes, there is combinatorics, but not too terrible. ~1000 packages, 4 ghc
versions -> 4000 <package> variables.

Ah, and I do not propose to actually support, for example,
ghc7.6-lens-4.13. If it fails to build, that it is dropped.
Some kind of automatization and integration with hydra would
be useful.

PS. Let's also discuss Emacs 'load-path'.

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