[Nix-dev] Re: experimental -J NUM_CORES option patch / looking for reviews

[Peter Simons]

Hi Isaac.

 >> [Nix needs the ability] to pass variables to an expression that do
 >> not modify the hash. $NUM_CORES is just one example of such a
 >> variable. Another one is $doCheck.
 >
 > Why, are you saying that testing should ever change the installed
 > program?

No, I meant to say the opposite. For the installed program, it doesn't
matter whether "make check" is run or not. Hence, the value of $doCheck
shouldn't be considered in the hash.

However, my point is not about $doCheck. My point is that Nix should
have the ability to pass a variable to an expression that's transparent
in terms of the hash (and thus the generated $out path). $NUM_CORES and
$doCheck are just examples for possible applications of that feature.


 > If we're using the model where derivations are defined to produce a
 > set of possible results that are functionally equivalent (because we
 > can't always force everything about the build process to be
 > deterministic in result-bytes), then it doesn't necessarily make
 > sense to say "doesn't change the hash" rather than "doesn't make the
 > behavior vary any more than it does between compiles without any
 > build-modifications"?

I am sorry, but I don't understand the meaning of this paragraph. Can
you please re-iterate this point?

Take care,
Peter