This fixes the "sliding window" principle:
0. Run packages: build = native; host = foreign; target = foreign;
1. Build packages: build = native; host = native; target = foreign;
2. Vanilla packages: build = native; host = native; target = native;
3. Vanilla packages: build = native; host = native; target = native;
n+3. ...
Each stage's build dependencies are resolved against the previous stage,
and the "foreigns" are shifted accordingly. Vanilla packages alone are
built against themsevles, since there are no more "foreign"s to shift away.
Before, build packages' build dependencies were resolved against
themselves:
0. Run packages: build = native; host = foreign; target = foreign;
1. Build packages: build = native; host = native; target = foreign;
2. Build packages: build = native; host = native; target = foreign;
n+2. ...
This is wrong because that principle is violated by the target
platform staying foreign.
This will change the hashes of many build packages and run packages, but
that is OK. This is an unavoidable cost of fixing cross compiling.
The cross compilation docs have been updated to reflect this fix.
If the flag enableIntegerSimple is true GHC will be build with the GPL-free but
slower integer-simple library instead of the faster but GPLed integer-gmp
library.
The attribute `pkgs.haskell.compiler.integer-simple."${ghcVersion}"` provides a
GHC compiler build with `integer-simple`.
Similarly, the attribute `pkgs.haskell.packages.integer-simple."${ghcVersion}"`
provides a package set supporting `integer-simple`.
Closes https://github.com/NixOS/nixpkgs/pull/22121.
Closes https://github.com/NixOS/nixpkgs/issues/5493.
The Nix search path is the established mechanism for specifying the
location of Nix expressions, so let's use it instead of adding another
environment variable.
Do so in the stdenv section where the other two are discussed. This can be
done without brining up cross-compilation by talking about build-time vs
run-time.
This patch add a new argument to Nixpkgs default expression named "overlays".
By default, the value of the argument is either taken from the environment variable `NIXPKGS_OVERLAYS`,
or from the directory `~/.nixpkgs/overlays/`. If the environment variable does not name a valid directory
then this mechanism would fallback on the home directory. If the home directory does not exists it will
fallback on an empty list of overlays.
The overlays directory should contain the list of extra Nixpkgs stages which would be used to extend the
content of Nixpkgs, with additional set of packages. The overlays, i-e directory, files, symbolic links
are used in alphabetical order.
The simplest overlay which extends Nixpkgs with nothing looks like:
```nix
self: super: {
}
```
More refined overlays can use `super` as the basis for building new packages, and `self` as a way to query
the final result of the fix-point.
An example of overlay which extends Nixpkgs with a small set of packages can be found at:
https://github.com/nbp/nixpkgs-mozilla/blob/nixpkgs-overlay/moz-overlay.nix
To use this file, checkout the repository and add a symbolic link to
the `moz-overlay.nix` file in `~/.nixpkgs/overlays` directory.
