Comment by mikepurvis

It's true— fundamentally, nothing about a build realizing partway through that it needs more stuff breaks the Nix philosophy, assuming the build is holding a hash for whatever it is it wants to pull so that everything stays hermetic. It's a bit annoying to not know upfront exactly what your build graph looks like but honestly it's not the worst— like, you already don't know how long each derivation is going to take.

In any case, the tvix devs have definitely understood the assignment on this and are making not only ifd a first class citizen, but also the larger issue of allowing the evaluation step to decompose, and for the decomposed pieces to run in parallel with each other and with builds— and that really is the game-changer, particularly with a cluster-backed build, to be able to start work immediately rather than waiting out a 30-60 second single-threaded eval.