On Fri, Dec 15, 2017 at 12:10 PM, Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org> wrote:
|  at this point in time Stackage works
|  hard to ensure that in any given package set, there is *exactly one*
|  version of any package. That's why Stackage aligns versions of core
|  packages to whatever ships with the GHC version the package set is
|  based on.

Ah. It follows that if Stackage wants to find a set of packages compatible with GHC-X, then it must pick precisely the version of bytestring that GHC-X depends on.  (I'm assuming here that GHC-X fixes a particular version, even though bytestring is reinstallable?  Certainly, a /distribution/ of GHC-X will do so.)

If meanwhile the bytestring author has decided to use a newer version of .cabal file syntax, then GHC-X is stuck with that.  Or would have to go back to an earlier version of bytestring, for which there might be material disadvantages.

That would make it hard to GHC to guarantee to downstream tools that it doesn't depend on any packages whose .cabal files use new syntax; which is where this thread started.

Hmm.  I wonder if I have understood this correctly.  Perhaps Michael would like to comment?



Stackage does in fact pin snapshots down to precisely one version of each package. And in the case of non-reinstallable packages, it ensures that those package's transitive dependency set are pinned to the same version that ships with GHC. I know there's work around making more package reinstallable, and the ghc package itself may have crossed that line now, but for the moment Stackage assumes that the ghc package and all its dependencies are non-reinstallable. Therefore bytestring, process, containers, transformers, and many more will be pinned in a Stackage snapshot.

Michael