[Haskell-beginners] Error when running `cabal install cabal-install`

[Brandon Allbery]

On Thu, Apr 23, 2015 at 5:12 PM, Peng Yu <pengyu.ut at gmail.com> wrote:

> I had problems with MacPorts for Python and Perl. The general
> suggestion is to stay way from MacPorts for managing packages offered
> by other language platforms.
>

Actually, the general suggestion there is stay away from *any* packaging
system for those languages --- because installing modules from both the
packaging system and the language's own package repository risks breaking
things with incompatible versions. I have had to help someone try to fix
their Debian install after they installed a Perl module that was
incompatible with dpkg; it's not just MacPorts, or even just Macs. (You can
screw up a Red Hat-ish system the same way by installing the wrong Python
module(s).) This is why Perl has perlbrew, Python has virtualenv, and Ruby
has rvm (and for ghc, both Cabal sandboxes and hsenv).

That said, a batteries-included setup like the Platform is usually an
exception because you want the compiler and the Platform packages to all
match, and you'll break things anyway if they don't: overriding modules
that comes with ghc or with the Platform (or Stackage if you use that) is
pretty much guaranteed "Cabal hell"). So get those all from the same place,
but other modules from wherever --- just don't try to upgrade or replace
those packages.

You can add a "constraint: <foo> installed" to ~/.cabal/config for each the
packages in the global package database to tell cabal-install not to touch
those libraries. Note that you will have to exclude the Cabal library
itself from that to install a newer cabal-install. This is actually
relatively safe, although there is the potential to break ghc-as-a-library
in obscure ways.

# # #

I think MP did end up skipping the latest Platform because the build system
changed in a way that's pretty much hostile to other package systems;
they've all had to come up with their own ways to build it instead, and our
Haskell maintainer did not have the time to work out how to adapt one of
them to a ports-based system or design his own alternative. The next
Platform release is backing off from that (it ended up being no easier than
the old way), so should be easier to update.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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