Provenance, aka “where did this attribute come from”

[Profpatsch]

One use-case for nixpkgs overlays I just had: We define an overlay of pkgs where we override and add some packages. I just went directly to nixpkgs to see the definition of a package, totally missing that we had overridden it locally.

It would be nice if there was a “stack trace” of sorts that can show how attributes “came to be”, aka where they were added/changed. I don’t know how feasable that is.

So, for example, file A defines:

{ foo = 32; }

file B says:

(import A) & { foo = self.foo + 10 }

And if I tell nickel: “show me the provenance of foo in B”:

A.ncl:1 foo = 32
B.ncl:1 foo = 42

or somesuch.

This uses the proposed self feature to do self-recursion with merges, but of course could be adapted to other ways of recursion.

[yannham]

This wouldn't be hard to do I think, using metavalues to store this information, without much overhead. Then the query sub-command could show this information along the value.

(import A) & { foo = self.foo + 10 }

As a side note, I don't see a way of interpreting this other than an infinite loop, but I get what you meant.

[Profpatsch]

Ah yes, self would refer to the foo I just define in this case, so I guess it would have to be super or something?

On Fri, Dec 4, 2020 at 1:41 PM Yann Hamdaoui [email protected] wrote:

This wouldn't be hard to do I think, using metavalues to store this information, without much overhead. Then the query sub-command could show this information along the value.

(import A) & { foo = self.foo + 10 }

As a side note, I don't see a way of interpreting this other than an infinite loop, but I get what you meant.

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