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

[Peter Simons]

Hi Marc,

the ability to utilize multi-processor machines is highly desirable.
Thank you for writing a patch that goes into this direction. 

I wonder, though, whether the approach of hard-coding know-how about the
$NUM_CORES variable into Nix itself is the way to go. IMHO, a more
general mechanism is required, namely 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. Some sites might
want to enable "make check" by default, others might not. In the general
case it's not obvious why a "make check"ed version of a given package
should have a different $out path than one that hasn't been checked.
Now, support for $doCheck could be hard-coded into Nix, too, of course,
but does it really need to be?

Take care,
Peter