Also allow "." as an installable to refer to the flake in the current
directory. E.g.
$ nix build .
will build 'provides.defaultPackage' in the flake in the current
directory.
Unlike file://<path>, this allows the path to be a dirty Git tree, so
nix build /path/to/flake:attr
is a convenient way to test building a local flake.
The general syntax for an installable is now
<flakeref>:<attrpath>. The attrpath is relative to the flake's
'provides.packages' or 'provides' if the former doesn't yield a
result. E.g.
$ nix build nixpkgs:hello
is equivalent to
$ nix build nixpkgs:packages.hello
Also, '<flakeref>:' can be omitted, in which case it defaults to
'nixpkgs', e.g.
$ nix build hello
This allows using an arbitrary "provides" attribute from the specified
flake. For example:
nix build --flake nixpkgs packages.hello
(Maybe provides.packages should be used for consistency...)
We want to encourage a brave new world of hermetic evaluation for
source-level reproducibility, so flakes should not poke around in the
filesystem outside of their explicit dependencies.
Note that the default installation source remains impure in that it
can refer to mutable flakes, so "nix build nixpkgs.hello" still works
(and fetches the latest nixpkgs, unless it has been pinned by the
user).
A problem with pure evaluation is that builtins.currentSystem is
unavailable. For the moment, I've hard-coded "x86_64-linux" in the
nixpkgs flake. Eventually, "system" should be a flake function
argument.
For example, github:edolstra/dwarffs is more-or-less equivalent to
https://github.com/edolstra/dwarffs.git. It's a much faster way to get
GitHub repositories: it fetches tarballs rather than entire Git
repositories. It also allows fetching specific revisions by hash
without specifying a ref (e.g. a branch name):
github:edolstra/dwarffs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553
Previously, plain derivation paths in the string context (e.g. those
that arose from builtins.storePath on a drv file, not those that arose
from accessing .drvPath of a derivation) were treated somewhat like
derivaiton paths derived from .drvPath, except their dependencies
weren't recursively added to the input set. With this change, such
plain derivation paths are simply treated as paths and added to the
source inputs set accordingly, simplifying context handling code and
removing the inconsistency. If drvPath-like behavior is desired, the
.drv file can be imported and then .drvPath can be accessed.
This is a backwards-incompatibility, but storePath is never used on
drv files within nixpkgs and almost never used elsewhere.
Jonas Chevalier
Linus Heckemann