[Nix-dev] Evaluation time vs build time

[Profpatsch]

On 17-05-31 08:07pm, Linus Heckemann wrote:
> On 31/05/17 18:01, Judson Lester wrote:
> As far as I understand it, it's mostly true — I believe it *is* actually
> possible, just strongly discouraged and absolutely not accepted in
> nixpkgs because (iiuc) of the mess of dependencies it can create.

That is true.

something like
```
let
  fooSrc = fechgit {};
  fooDerivation = import "${fooSrc}/default.nix"
in
  fooDerivation
```

will build the fooSrc derivation at *evaluation time*
(blocking the evaluator) before building the derivation
imported from the fooSrc derivation output.

Because the hash of `fooSrc` has to be given,
the default.nix in `fooSrc` is also fixed-input.

It is strongly discouraged in nixpkgs,
though I sometimes use it for private projects.

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