Add builtins.dateFormat function

[doronbehar]

Is your feature request related to a problem? Please describe.

When composing a flake.nix, often we need to build a dependency from a flake input that has flake = false;. Defining a Nix expression for such a package requires setting pname and version, as always. If one fetches a non release version of this flake input, you may want to set the version to unstable-<last-commit-date>, as we do in Nixpkgs. It's natural to take this date from the lastModified json key of the input in flake.lock. However, this json key is an epoch number, and not human readable.

Describe the solution you'd like
I think ideally, I could use a pure Nix function as so:

lockFile = builtins.fromJSON (builtins.readFile ./flake.lock);
version = builtins.dateFormat lockFile.nodes.${myDep}.locked.lastModified;

Describe alternatives you've considered

Get this date via:

epochDate2human = epochDate: builtins.readFile (pkgs.runCommand "epochDateConvertion" {} ''
  printf '%(%F)T' ${builtins.toString epochDate} > $out
'');

But this is of course IFD, which is forbidden in restricted mode, and when evaluating packages for commands like nix search.

[roberth]

Perhaps we should return the whole sourceInfo instead of just a path when flake = false.

[doronbehar]

Cannot find documentation for sourceInfo... Will that help me regarding reading flake.lock? or also include a kind of date attribute?

[roberth]

It's an attribute of the flake = true flake inputs containing the source path and limited metadata like lastModified. Basically a fetchTree result iiuc.

[Kranzes]

You can also do something like this for now builtins.concatStringsSep "-" (builtins.match "(.{4})(.{2})(.{2}).*" inputs.self.lastModifiedDate)

[doronbehar]

You can also do something like this for now builtins.concatStringsSep "-" (builtins.match "(.{4})(.{2})(.{2}).*" inputs.self.lastModifiedDate)

Hmm not bad, though not very explanatory:

"1549-46-64"

[roberth]

Another time related "issue" is

• Parse TOML timestamps as strings #8120

We might say that time logic is creeping in and we need a better story for it. However, time handling libraries are notoriously hard to implement, suggesting that we'd be better off pinning a nix library such as the Nixpkgs lib, so that we don't risk tying reproducibility to our inevitable time handling bugs.

[roberth]

"1549-46-64"

lastModified or lastModifiedDate?

[doronbehar]

"1549-46-64"

lastModified or lastModifiedDate?

Oh I see, it's not the lastModified json key from flake.lock - it's a key of flake input argument. Thanks for noticing, indeed it works! Thanks a lot @Kranzes 💚 .

[Kranzes]

lastModified/lastModifiedDate/rev/shortRev are just coming from an input's sourceInfo attr

[balsoft]

Here's my take on this in pure Nix: https://github.com/balsoft/Flockenzeit

  version = with flockenzeit.lib.splitSecondsSinceEpoch {} lastModified; "unstable-${F}"; # %F from `man date'