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I'm attempting to start a new project using the Snap web framework. I used snap init to get my basic skeleton working. I also put http://www.stackage.org/lts/cabal.config next to my .cabal file. I didn't uncomment the line to use Stackage exclusively. So I tried to build and it failed and couldn't find the version of lens required by my .cabal file. The cabal.config file from Stackage specifies a version of lens that is not the same as the one in my .cabal file. So I deleted every constraint from my package list and did the usual cabal install --only-dep -j8 --enable-test and it worked!

However, I have always been told that package versions should be constrained. So when working with Stackage is it okay to leave package versions unconstrained? Should I downgrade my packages to the ones available in Stackage instead?

As far as I understand a cabal.config file specifies a set of dependencies with the specific versions that satisfy dependencies, so how does Stackage work? Is it just a subset of packages from Hackage that are proven to be compatible? Do they host their own packages or rely on Hackage for downloads?

Thanks in advance :)

Juan Pablo Santos
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1 Answers1

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Both options are available. The default option is what you did, and still goes to hackage to get the packages. You just added a filter to your cabal that prevents you from using any version of a package included in Stackage that was not tested to work together with all of the other packages.

The other option is to simply point your cabal repo to a Stackage url, and then you will download packages directly from the Stackage server. That server will only serve packages that are known to work together, so there is no need for additional constraints in your cabal file. I actually prefer this way of working.

In both cases, if you have additional constraints in your cabal file that are incompatible with the Stackage restrictions, your build will fail. If you use the first option, you will get dependency conflicts. When using the second option, the Stackage server will simply report that it does not have that specific package/version.

kdkeyser
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  • Follow up, so should I add Stackage-compatible constraints to my .cabal file? – Juan Pablo Santos Feb 03 '15 at 23:55
  • If you are only going to be building against Stackage, I don't see the point of adding constraints to your .cabal files: the chosen Stackage snapshot provides the whole description of the package set and its constraints. If you want other people to be able to build your package against Hackage, it would make sense to keep the constraints based on the API's that you use of the packages. – kdkeyser Feb 04 '15 at 08:47