[Nix-dev] Toward a Purer stdenv

[Shea Levy]

Hello,

I've been thinking about creating a nixpkgs branch to find and eliminate 
some of the remaining impurities in the Linux stdenv. Before I get 
started, I wanted to poll the list with a few questions that will shape 
how I go about things.

1. What impurities does the current stdenv have? I can think of three 
off of the top of my head: timestamps in files, kernel version (e.g. via 
uname), and /bin/sh.

2. What kind of testing should I set up to see that my changes actually 
remove impurities?

3. When might it be desirable to have impurities? I can think of three 
cases: When knowledge of the performance of the system could potentially 
speed up a build (e.g. knowing how many cores the CPU has for parallel 
building), when knowledge of the performance of the system is necessary 
for a test (e.g. checking that running a certain tool completes its task 
in the right amount of time), and when a build would otherwise fail. My 
current view is that the first two cases are OK as long as the 
impurities (preferably provably) don't affect the output (think 
unsafePerformIO), and that the for the third case we should attempt to 
fix the build system in question if possible and should explicitly 
include whatever system information is being accessed as a dependency if 
not. Ideally our build system would give us ways to purposely allow 
impurities when they are of the first two types, such as the 
enableParallelBuilding variable the generic builder currently recognizes.

Cheers,
Shea Levy