@rutrum isn't it because of the fear of import from derivation?
this post was submitted on 02 Aug 2025
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Could you say a bit more about that? I was actually wondering about the exact same thing as OPs question. Is the performance so bad that it doesn't even make sense to consider this as an option? Idk, just like OP I feel like it's such an obvious use case that it really should be a thing.
@QuizzaciousOtter
You can then no longer split pure evaluation and build as you need to build an undefined amount of things to finish the first evaluation. This is especially worse for Hydra, for nixos systems it usually works. Especially when bootstrapping is involved this can make the initial eval take almost days.
This post exactly expresses my thoughts from some months ago. However, I wasn't motivated enough to actually post it. Thanks for reading in my mind and asking that for me, I appreciate it.
Nix builds go through 2 phases, first downloading the dependencies into the nix store (which can also go through a nix build) and then executing the builder with access to the nix store but without network access. This is done for purity because network access can lead to non-reproducible builds.
Most languages and tools are complex but can output URLs to their depdencies or download them, but that cannot happen during the build (no network access). So, either a tool predownloads stuff and they can be put into the nix store by the "simple" nix derivation, or the tool (pip, docker compose, cargo,...) has to be rewritten either partially or completely in nix. The latter is much harder and in most cases would just lead to repeating the work. Nix may be a functional language but it's missing important language features like static typing.
There's work being done to make this possible in the future https://fzakaria.com/2025/03/10/an-early-look-at-nix-dynamic-derivations