[Nix-dev] Will Haskell-ng and hackage2nix allow building *any* versions of deps?

[Mateusz Kowalczyk]

On 02/22/2015 12:04 PM, Peter Simons wrote:
> Hi Cody,
> 
>  > haskellngPackages doesn't seem to have all versions of all dependencies...
> 
> I'm not sure what you mean by "all versions of all dependencies". Dependencies
> of what exactly? The way I understand the term, "dependency" has meaning only
> as a relationship between two packages, i.e. "transformers" is a dependency of
> "mtl". I'm not sure how to apply that interpretation to your question?
> 
> If you're asking why hackage-packages.nix doesn't contain every single version
> of every package registered on Hackage, then the answer is because that would
> be a database containing well over 50,000 packages, the vast majority of which
> no-one will ever care about (nor will they ever compile). So it seems pointless
> to distribute all that stuff as part of Nixpkgs.
> 
> Best regards,
> Peter
> 
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A related question: is there a way to generate a package list containing
exactly the versions of dependencies we require?

Say we have a package with cabal file

A == 1.0
B > 2.0
C < 0.5

but the latest Hackage has versions lower and higher &c and that's what
is in nixpkgs. I found myself wishing that there was an easy to just say
‘give me a set of packages that's valid from cabal's point of view’
rather than what we currently tend to do which is ‘include latest
versions of everything, jailbreak and hope it works’ which is fine for
nixpkgs but subpar if we just want that one project working but it
requires specific versions of dependencies.

-- 
Mateusz K.