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

[Roman Cheplyaka]

* 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)

Roman