Optimise IFD?

[L-as]

Currently, AFAICT, haskell.nix makes extensive use of IFD in the form of essentially importing the materialisation.
This is problematic because of many obvious reasons:

• IFD blocks evaluation, so if you have (import x.outPath) + (import y.outPath), they will be built sequentially, rather than in parallel, which is a huge issue.
• Hydra doesn't work well with it.

A very related issue on GH is NixOS/rfcs#109

By putting all IFD into a separate derivation, e.g. Hydra could build this before building everything else, thus ensuring correct functioning. It would also fix the issue with parallelism.
This seems like a big undertaking to me however?

However, it seems to me like the problem also lies with Nix. There doesn't seem to be a good reason for the sequential behavior of IFD, see NixOS/nix#6299.

[michaelpj]

By putting all IFD into a separate derivation

What does this mean? IFD is, necessarily, "in a separate derivation", it's "import-from-derivation" after all. I really don't know what you're getting at here.

[L-as]

one derivation. The RFC elaborates more on the approach.

[stale]

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