Stability promise, reprodubility properties #10694
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documentation
idea approved
The given proposal has been discussed and approved by the Nix team. An implementation is welcome.
Problem
It's unclear what are Nix's stability and reproducibility promises, or how successfully we've kept them.
E.g. question raised by @physics-enthusiast
But also what do we promise about other components than the language?
E.g. also guarantees about stability of store level functionality.
Proposal
A page about stability and reproducibility.
It should answer the above questions, and contain some of the here:
They are meant to be stable, except for bugfixes and functions behind a feature flag.
We have on this occasion allowed an existing function to be more strict (iirc by mistake, initially) for the benefit of better error messages, but only because it does not produce a different result (denotationally, and modulo errors and other bottoms), it does not produce real world problems, and reverting the change would regress the quality of error messages.
Another instance where behavior has changed is
fetchGit
, because it had reproducibility problems. That was a bug.It should generally be ok to introduce more laziness, except of course in primops that are about adding strictness, and it is also not ok to add laziness at the cost of other behaviors (i.e. laziness is not an excuse to make changes to other behaviors such as the denotational semantics).
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