lib/flakes: move eachSystem from flake-utils

lib/flakes.nix

@@ -19,4 +19,57 @@ rec {
     in
     self;
 
+
+  /* Builds a map from <attr>=value to <attr>.<system>=value for each system,
+     except for the `hydraJobs` attribute, where it maps the inner attributes,
+     from hydraJobs.<attr>=value to hydraJobs.<attr>.<system>=value.
+
+     Example:
+       eachSystem [ "x86_64-linux" "aarch64-linux" ] (system: { hello = 42; })
+       => { hello = { aarch64-linux = 42; x86_64-linux = 42; }; }
+  */
+  eachSystem = systems: f:
+    let
+      # 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 lib.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:
+            let
+              appendSystem = key: system: ret:
+                if key == "hydraJobs"
+                  then (pushDownSystem system (attrs.hydraJobs or {}) ret.hydraJobs)
+                  else { ${system} = ret.${key}; };
+            in attrs //
+              {
+                ${key} = (attrs.${key} or { })
+                  // (appendSystem key system ret);
+              }
+          ;
+        in
+        builtins.foldl' op attrs (builtins.attrNames ret);
+    in
+    builtins.foldl' op { } systems;
 }