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

On 14 November 2012 20:35, Roman Cheplyaka wrote:

* Erik Hesselink [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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