fyi, below you can find an expanded list of related links.

I do appreciate the new sandboxing feature in 1.18 and I think it is a great feature, but it is not addressing the core issue here: adding support for multiple instances of the same version. It also seems to me that supporting this would be a relatively low hanging fruit that could have a big impact to the whole ecosystem once addressed. So why not address it? I don't have enough exposure to Ghc or Cabal to make a difference here, now (what is needed is an elaboration of the issues together with a minimal solution), but I would be willing to participate in a supporting role (testing, reviewing, and minor bug fixes).


GSoC 2012 - Enable GHC to use multiple instances of a package for compilation - Philipp Schuster
http://www.google-melange.com/gsoc/proposal/review/google/gsoc2012/phischu/1
http://www.google-melange.com/gsoc/project/google/gsoc2012/phischu/19001
https://github.com/phischu/cabal

GHC Commentary
http://ghc.haskell.org/trac/ghc/wiki/Commentary/Packages/MultiInstances
http://ghc.haskell.org/trac/ghc/wiki/Commentary/Packages
http://ghc.haskell.org/trac/ghc/wiki/Commentary/GSoCMultipleInstances

Mailing list
http://comments.gmane.org/gmane.comp.lang.haskell.ghc.devel/443
http://markmail.org/message/4qvegvx32lhlo66g#query:+page:1+mid:bwdgykv4g2hzqg5t+state:results



On Thu, Sep 5, 2013 at 4:33 AM, Simon Peyton-Jones <simonpj@microsoft.com> wrote:
| > Can I ask what the Cabal team's position is with respect to the
| question of allowing the same package to be installed several times,
| each compiled against a different collection of dependencies?
|
| I think that we all agree that in the long term a Nix-like package
| database is the ideal solution to the "Cabal hell" problem (I even
| mentioned this in the "Future Work" section of the post you linked).
| However, it seems to be much harder to implement than sandboxes.

I have not read the full GSoC page (thanks for that link), but are you all convinced that it *needs* to be that hard?

The fundamental idea is, after all, so simple!  (I could elaborate.)  Perhaps the GSoc project was trying to be more ambitious.

Or maybe I'm simply under-estimating.  But fundamentally it seems simple, so I'm suggesting we should perhaps try a bit harder to ferret out the underlying simplicity rather than assuming it must be hard.

Simon


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