rethrow primop

[roberth]

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

The error message can not be touched

• tryEval must be independent of evaluation order #8688

However, in many cases, users just want to add some info to the message.

In other cases, they may want to combine messages.

Both can be done without making evaluation order dependent.

Describe the solution you'd like

builtins.rethrow such that

builtins.rethrow
  { a = "one"; b = throw "foo"; c = throw "bar"; }
  ({ a ? null, b, c }:
    assert a == null;
    "Errors are ${b} and ${c}"
  )
  v

causes the error

error: Errors are foo and bar

If they care about the value when successful, they can use a let binding to retrieve it.

Describe alternatives you've considered

• Alternatively or additionally, a list could be supported instead of an attrset.

• We could allow arbitrary values in the exception message, which would be printValued if they bubble up without getting changed back to a string.

• Poison pill string context #8388

• Structured markup for throw and trace messages that make Nix more robust #7795

Additional context

Priorities

Add 👍 to issues you find important.

[roberth]

How should the stack trace behave?
It could be made similarly "first class":

withTrace (trace: msg: ...) v

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-07-31-nix-team-meeting-minutes-76/31486/1

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-09-22-nix-team-meeting-minutes-88/33343/1

[infinisil]

I remember talking with Robert about how the proposed rethrow here doesn't entirely work out as described, though that something like it should work. So I don't think it should be blindly implemented as shown here, but rather someone should spend some time to think about how exactly it should work first. Just as a note to anybody wanting to go for this.

Since this isn't a high-priority feature, I won't be attempting this myself for now though (and I hope Robert won't either), there's better things to focus on imo :)

[fricklerhandwerk]

Triaged in Nix team meeting 2023-09-22:

idea approved to add as experimental feature