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.
This makes 'nix copy' and 'nix path-info' work on .drv store
paths. Removing special treatment of .drv files seems the most
future-proof approach given the possible removal of .drv files in the
future.
Note that 'nix build' will still build (rather than substitute) .drv
paths due to the unfortunate overloading in Store::buildPaths().
This doesn't work in read-only mode, ensuring that operations like
nix path-info --store https://cache.nixos.org -S nixpkgs.hello
(asking for the closure size of nixpkgs.hello in cache.nixos.org) work
when nixpkgs.hello doesn't exist in the local store.
On second though this was annoying. E.g. "nix log nixpkgs.hello" would
build/download Hello first, even though the log can be fetched
directly from the binary cache.
May need to revisit this.
So for instance "nix copy --to ... nixpkgs.hello" will build
nixpkgs.hello first. It's debatable whether this is a good idea. It
seems desirable for commands like "nix copy" but maybe not for
commands like "nix path-info".
Thus
$ nix build -f foo.nix
will build foo.nix.
And
$ nix build
will build default.nix. However, this may not be a good idea because
it's kind of inconsistent, given that "nix build foo" will build the
"foo" attribute from the default installation source (i.e. the
synthesis of $NIX_PATH), rather than ./default.nix. So I may revert
this.
That is, unless --file is specified, the Nix search path is
synthesized into an attribute set. Thus you can say
$ nix build nixpkgs.hello
assuming $NIX_PATH contains an entry of the form "nixpkgs=...". This
is more verbose than
$ nix build hello
but is less ambiguous.
Jörg Thalheim