stack-to-nix: Better default.nix template

[rvl]

The stack-to-nix template (default.nix) which was added in #20 could be improved.

1. haskell.nix should be pinned to a git rev with sha256 hash. It could use the exact same haskell.nix pin that nix-tools was built with.
2. nixpkgs should be also be pinned -- otherwise the build may not work.
3. The file should be generated with hnix, rather than hardcoded strings.
4. It would be good to have some comments in this file (although that may conflict with point 3 -- maybe just prepend comments to the top of the file).

[fendor]

Regarding pinning of haskell.nix does it only make sense to pin the revision to the same as nix-tools was built with?
Is the hackage index influeced by that version pinning? E.g., if you do not update nix-tools for a long period of time, are some packages unknown that had been released since then? Or is the hackage index independent of nix.tools?

Pinning of nixpkgs, is pinning to the current revision of nixpkgs sensible?
Would an additional CLI parameter make sense?

[angerman]

@fendor in #210 I'm trying to move haskell.nix into an overlay, at which point you can pin your own nixpkgs version as you like. It will also remove the iohk-nix dependency for any more fancy needs.

On a more general note, there is some haskell.nix -> nix-tools dependency. As haskell.nix may require features of newer nix-tools, but those will be bumped in haskell.nix as needed. Hence pinning haskell.nix should be sufficient.

However, haskell.nix will bring in hackage and stackage, as such, if you require newer packages (or stackage snapshots), you are best advised to just update haskell.nix.

[stale]

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