Subject: Re: Cabal, Haskell Platform
From: "Edward Z. Yang" <[email-masked]>
To: Simon Peyton Jones <[email-masked]>
Cc: Gershom B <[email-masked]>, "\"Duncan Coutts ([email-masked])\"" <[email-masked]>, Ben Gamari <[email-masked]>, Austin Seipp <[email-masked]>
Date: Sat, 31 Oct 2015 22:52:11 +0000

Hello Simon,

> ·         GHC has for some time had the hooks to install multiple
> instances of the same library without conflict, but Cabal does not (or
> did not).  I know Duncan was planning to do this.   Will the Cabal
> that comes with HP 8.0 have this ability?

A month ago I wrote a blog post attempting to poll the community
on whether or not we should wait for Duncan's patchset, or directly
forge ahead and enable no-reinstall.  There was subsequent discussion
on Reddit.

    https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fblog.ezyang.com%2f2015%2f09%2fis-no-reinstall-cabal-coming-to-ghc-8%2f&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c72264ec38b0442a410c808d2e245eb10%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=FZt6ptLhLCOYUJDIuzvS6JEuDjuCMy1V1WrHBDoHnEo%3d
    https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fwww.reddit.com%2fr%2fhaskell%2fcomments%2f3lilqh%2fis_noreinstall_cabal_coming_to_ghc_80%2f&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c72264ec38b0442a410c808d2e245eb10%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=IotCx6hShbde5tIohJdtCok7epWc3TjVLn3INTZD9uk%3d

I decide to look at the discussion again, and I think there
is consensus to implement this plan for HP 8.0:

    1. We add a new flag, --enable-multiple-reinstalls, which turns
    on the "no reinstall" behavior for GHC 7.10 and later (which
    we do have a patch for).

    2. When the package plan would cause conflicts, we instead
    ask the user if they would like to add the flag
    --enable-multiple-reinstalls (in place of where we suggest
    --force-reinstalls)

I don't think the patch is hard and I will write it when I get a chance.

Edward