Bumping Hackage and Stackage snapshots

haskell.nix relies on some generated data providing information about packages in Hackage and Stackage snapshots. These are kept in hackage.nix and stackage.nix respectively. If your project depends on a Hackage package, then the hackage.nix revision used must be new enough to contain that, and likewise for Stackage snaphots and stackage.nix.

Updating and pinning hackage.nix and stackage.nix

haskell.nix pins particular revisions of these repositories internally, both for our own usage in testing, and so that users have a sensible default when getting started. These revisions are updated nightly, so you can get newer revisions of hackage.nix and stackage.nix by updating your revision of haskell.nix itself.

However, this exposes you to changes in haskell.nix which you may not want, such as changes that force compiler rebuilds, or the occasional bug. Instead, you can pin hackage.nix and stackage.nix independently. For example:

let
  # You can use a tool like `niv` to manage this boilerplate
  hackageSrc = builtins.fetchTarball https://github.com/input-output-hk/hackage.nix/archive/master.tar.gz;
  stackageSrc = builtins.fetchTarball https://github.com/input-output-hk/stackage.nix/archive/master.tar.gz;
  haskellSrc = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz;

  haskellNix = import haskellSrc {
    # This allows you to override the pins used by `haskell.nix` internally
    sourcesOverride = {
      hackage = hackageSrc;
      stackage = stackageSrc;
    };
  };
in ...

This way you can change the revisions of hackage.nix and stackage.nix without changing haskell.nix.

However, bear in mind that Stackage refers to Hackage, so your Stackage pin should never be newer than your Hackage pin.