I'm in favor of adding support to Cabal to allow for this situation. However: I highly doubt this will be the panacea as predicted. It's already a huge source of confusion for people using GHCi what they get messages about "ByteString is not ByteString." In fact, this confusion is so prevalent that I wrote a blog post about it in September[1].
I strongly believe that the best end-user experience comes from having a single mapping from package name to actually installed package/version/dependencies. I'd go even farther and say that users would be well served by having a single mapping from module name to installed module. In fact, Adam Bergmark even created an issue[2] to look into adding support to Stackage to start enforcing this situation, though the response to a blog post I wrote on that[3] implies that people are not so interested in addressing that problem. So my word of warning here is: if we simply throw multiple package/version/dependency combinations at users via cabal, there's a good chance that we'll do more harm than good. [1] http://www.yesodweb.com/blog/2014/09/woes-multiple-package-versions [2] https://github.com/fpco/stackage/issues/416 [3] http://www.yesodweb.com/blog/2014/02/module-name-conflicts On Mon, Mar 23, 2015 at 10:45 AM Simon Peyton Jones <simo...@microsoft.com> wrote: > Dear Cabal developers > > You'll probably have seen the thread about the Haskell Platform. > > Among other things, this point arose: > > | Another thing we should fix is the (now false) impression that HP gets > in > | the way of installing other packages and versions due to cabal hell. > > People mean different things by "cabal hell", but the inability to > simultaneously install multiple versions of the same package, > compiled against different dependencies > is certainly one of them, and I think it is the one that Yitzchak is > referring to here. > > But some time now GHC has allowed multiple versions of the same package > (compiled against different dependencies) to be installed simultaneously. > So all we need to do is to fix Cabal to allow it too, and thereby kill of a > huge class of cabal-hell problems at one blow. > > But time has passed and it hasn't happened. Is this because I'm > misunderstanding? Or because it is harder than I think? Or because there > are much bigger problems? Or because there is insufficient effort > available? Or what? > > Unless I'm way off beam, this "multiple installations of the same package" > thing has been a huge pain forever, and the solution is within our grasp. > What's stopping us grasping it? > > Simon > > _______________________________________________ > Libraries mailing list > librar...@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries >
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