eachSystem: push down `system` as far down as possible for Hydra jobs (#46)

default.nix

@@ -66,16 +66,46 @@ let
   #
   eachSystem = systems: f:
     let
+      # Taken from <nixpkgs/lib/attrsets.nix>
+      isDerivation = x: builtins.isAttrs x && x ? type && x.type == "derivation";
+
+      # Used to match Hydra's convention of how to define jobs. Basically transforms
+      #
+      #     hydraJobs = {
+      #       hello = <derivation>;
+      #       haskellPackages.aeson = <derivation>;
+      #     }
+      #
+      # to
+      #
+      #     hydraJobs = {
+      #       hello.x86_64-linux = <derivation>;
+      #       haskellPackages.aeson.x86_64-linux = <derivation>;
+      #     }
+      #
+      # if the given flake does `eachSystem [ "x86_64-linux" ] { ... }`.
+      pushDownSystem = system: merged:
+        builtins.mapAttrs
+          (name: value:
+            if ! (builtins.isAttrs value) then value
+            else if isDerivation value then (merged.${name} or {}) // { ${system} = value; }
+            else pushDownSystem system (merged.${name} or {}) value);
+
+      # Merge together the outputs for all systems.
       op = attrs: system:
         let
           ret = f system;
           op = attrs: key:
-            attrs //
+            let
-            {
+              appendSystem = key: system: ret:
-              ${key} = (attrs.${key} or { }) // (if key == "hydraJobs"
+                if key == "hydraJobs"
-              then builtins.mapAttrs (name: value: (attrs.hydraJobs.${name} or { }) // { ${system} = value; }) ret.hydraJobs
+                  then (pushDownSystem system (attrs.hydraJobs or {}) ret.hydraJobs)
-              else { ${system} = ret.${key}; });
+                  else { ${system} = ret.${key}; };
-            }
+            in attrs //
+              {
+                ${key} = (attrs.${key} or { })
+                  // (appendSystem key system ret);
+              }
           ;
         in
         builtins.foldl' op attrs (builtins.attrNames ret);