Change idiom for overriding Dhall package version

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.

1 files changed, 5 insertions, 13 deletions

diff --git a/pkgs/top-level/dhall-packages.nix b/pkgs/top-level/dhall-packages.nix
index 9d4801e9dc5cc..c1c2c5f0e3bbc 100644
--- a/pkgs/top-level/dhall-packages.nix
+++ b/pkgs/top-level/dhall-packages.nix
@@ -8,16 +8,6 @@ let
     let
       callPackage = newScope self;
 
-      prefer = version: path:
-        let
-          packages = callPackage path { };
-
-        in
-          packages."${version}".overrideAttrs (_: {
-              passthru = packages;
-            }
-          );
-
       buildDhallPackage =
         callPackage ../development/interpreters/dhall/build-dhall-package.nix { };
 
@@ -34,14 +24,16 @@ let
           buildDhallDirectoryPackage
         ;
 
+        lib = import ../development/dhall-modules/lib.nix { inherit lib; };
+
         dhall-kubernetes =
-          prefer "3.0.0" ../development/dhall-modules/dhall-kubernetes.nix;
+          callPackage ../development/dhall-modules/dhall-kubernetes.nix { };
 
         dhall-packages =
-          prefer "0.11.1" ../development/dhall-modules/dhall-packages.nix;
+          callPackage ../development/dhall-modules/dhall-packages.nix { };
 
         Prelude =
-          prefer "13.0.0" ../development/dhall-modules/Prelude.nix;
+          callPackage ../development/dhall-modules/Prelude.nix { };
       };
 
 in