This adds a way to specify the `evalSystem` or `evalPackages` explicitly when calling the `project` functions.
Currently if we want to make a `flake` that supports multiple systems we have few options:
* Require builders for all the supported systems (even just for `nix flake show`).
* Pass `--impure` so that haskell.nix can see `builtins.currentSystem` to set up `pkgs.evalPackages` to use that. Unfortunately this prevents nix from caching some of the work it does and often results in it recalculating for each supported system when it would otherwise be cached and take no time at all.
* Add an overlay to replace `evalPackages`. This works, but it is not straight forward.
* Materialize the nix files for the project.
This change allows `evalSystem = "x86_64-linux";` to be passed telling `haskell.nix` to run `cabal` and `nix-tools` on that system. The user will have to have a builder for that system, but does not need to have builders for the others (unless building outputs for them).
* Include dependencies of haskell.nix that were tracked in `nix/sources.json`
as flake inputs (`flake.lock` replaces `nix/sources.json`).
* Use `flake-compat` to continue to provide a compatible interface for non
flake projects.
Co-authored-by: Alexander Bantyev <balsoft@balsoft.ru>
Co-authored-by: Shea Levy <shea@shealevy.com>
The `hix` tools are wrappers for the various `nix` tools that
use `haskell.nix` without the need to add any `.nix` files.
This is useful for:
* A quick way to try out haskell.nix for new users.
* Using haskell.nix to work on projects that do not have
`.nix` files.
* Testing to see if `haskell.nix` can build a project.
* Making `flake` and `non flake` configurations to check `haskell.nix`
treats them the same.
The configuration arguments for `Hix` can be (from highest precedence to lowest):
* Passed on the command line with `--arg` (or `--argstr` for string args).
* Placed in `nix/hix.nix` file in the project dir.
* Placed in `~/.config/hix/hix.conf`
Boilerplate `default.nix`, `shell.nix` and `flake.nix` files can be added to a
a project with a `nix/hix.nix` file to make it work with the standard `Nix`
tools.
