[Haskell-cafe] How to determine correct dependency versions for a library?

[Ivan Lazar Miljenovic]

On 14 November 2012 20:35, Roman Cheplyaka <roma at ro-che.info> wrote:
> * Erik Hesselink <hesselink at gmail.com> [2012-11-12 20:58:17+0100]
>> And I believe the last base changes included a change to 'catch' which
>> would have broken a lot of packages. The Num changes also caused a lot of
>> code changes, and there were probably more I don't remember.
>
> Even if so, upper bounds don't prevent these errors. Cabal can't install
> an older base for you.
>
> (I'm aware that GHC once shipped two versions of base, and dependency
> bounds were actually useful then. But that's not the case nowadays, as
> we see.)
>
> For example, here's what I get when I try to install virthualenv with
> GHC 7.6.1:
>
>   % cabal install virthualenv
>   Resolving dependencies...
>   cabal: Could not resolve dependencies:
>   trying: virthualenv-0.2.1
>   rejecting: base-3.0.3.2, 3.0.3.1 (global constraint requires installed
>   instance)
>   rejecting: base-4.6.0.0/installed-eac... (conflict: virthualenv =>
>   base>=4.2.0.0 && <4.6)
>   rejecting: base-4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0,
>   4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0 (global constraint requires
>   installed instance)

Doesn't this prevent the error of "this package won't build" (even if
the error message doesn't precisely say that)?

>
> Roman
>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com