The `buildDhall{Directory,GitHub}Package` utilities now take an
optional `document` argument for generating documentation using
`dhall-docs`. The documentation is stored underneath the `./docs`
subdirectory of the build product.
Before this change, a Dhall package like the Prelude would be
encoded as a record with one field per supported version. Then
downstream packages would specify which package to override
by selecting a different record field.
The problem with that approach is that it did not provide an
easy way to override a package to a version other than the default
ones supplied by Nixpkgs. Normally you would use the `.override`
method for this purpose, but the `override` method added by
`buildDhall{Directory,GitHub}Package` is clobbered by the
`override` method added by `callPackage` in
`./pkgs/top-level/dhall-packages.nix`.
The solution is to add a separate `.overridePackage` method which is
essentially the exact same as `.override`, except that it is no
longer clobbered by `callPackage`. This `.overridePackage` method
allows one to override the arguments supplied to
`buildDhall{Directory,GitHub}Package`, making it easier to specify
package versions outside of the ones supported by Nixpkgs..
This also includes a change to only build one (preferred) version of each
package (instead of multiple supported versions per package), in order to
minimize the maintenance burden for the Dhall package set.
The motivation for this change is to enable a new Dhall command-line
utility called `dhall-to-nixpkgs` which converts Dhall packages to
buildable Nix packages. You can think of `dhall-to-nixpkgs` as the
Dhall analog of `cabal2nix`.
You can find the matching pull request for `dhall-to-nixpkgs` here:
https://github.com/dhall-lang/dhall-haskell/pull/1826
The two main changes required to support `dhall-to-nixpkgs` are:
* Two new `buildDhall{Directory,GitHub}Package` utilities are added
`dhall-to-nixpkgs` uses these in the generated output
* `pkgs.dhallPackages` now selects a default version for each package
using the `prefer` utility
All other versions are still buildable via a `passthru` attribute
