Move property processing, type checking, and merge code into a function

This makes the relationship between property types clearer, and more
importantly will let option types parameterized by other option types
reuse the code for delegated type checking and merging.

lib/modules.nix

@@ -106,12 +106,9 @@ rec {
               else []
             ) configs);
           nrOptions = count (m: isOption m.options) decls;
-          # Process mkMerge and mkIf properties.
+          # Extract the definitions for this loc
-          defns' = concatMap (m:
+          defns' = map (m: { inherit (m) file; value = m.config.${name}; })
-            if m.config ? ${name}
+            (filter (m: m.config ? ${name}) configs);
-              then map (m': { inherit (m) file; value = m'; }) (dischargeProperties m.config.${name})
-              else []
-            ) configs;
         in
           if nrOptions == length decls then
             let opt = fixupOptionType loc (mergeOptionDecls loc decls);
@@ -177,27 +174,17 @@ rec {
      config value. */
   evalOptionValue = loc: opt: defs:
     let
-      # Process mkOverride properties, adding in the default
+      # Add in the default value for this option, if any.
-      # value specified in the option declaration (if any).
+      defs' = (optional (opt ? default)
-      defsFinal' = filterOverrides
+        { file = head opt.declarations; value = mkOptionDefault opt.default; }) ++ defs;
-        ((if opt ? default then [{ file = head opt.declarations; value = mkOptionDefault opt.default; }] else []) ++ defs);
+      # Handle properties, check types, and merge everything together
-      # Sort mkOrder properties.
+      inherit (mergeDefinitions loc opt.type defs') defsFinal mergedValue;
-      defsFinal =
-        # Avoid sorting if we don't have to.
-        if any (def: def.value._type or "" == "order") defsFinal'
-        then sortProperties defsFinal'
-        else defsFinal';
       files = map (def: def.file) defsFinal;
-      # Type-check the remaining definitions, and merge them if
-      # possible.
       merged =
         if defsFinal == [] then
           throw "The option `${showOption loc}' is used but not defined."
         else
-          fold (def: res:
+          mergedValue;
-            if opt.type.check def.value then res
-            else throw "The option value `${showOption loc}' in `${def.file}' is not a ${opt.type.name}.")
-            (opt.type.merge loc defsFinal) defsFinal;
       # Finally, apply the ‘apply’ function to the merged
       # value.  This allows options to yield a value computed
       # from the definitions.
@@ -209,6 +196,33 @@ rec {
         inherit files;
       };
 
+    # Merge definitions of a value of a given type
+    mergeDefinitions = loc: type: defs: rec {
+      defsFinal =
+        let
+          # Process mkMerge and mkIf properties
+          discharged = concatMap (m:
+            map (value: { inherit (m) file; inherit value; }) (dischargeProperties m.value)
+          ) defs;
+
+          # Process mkOverride properties
+          overridden = filterOverrides discharged;
+
+          # Sort mkOrder properties
+          sorted =
+            # Avoid sorting if we don't have to.
+            if any (def: def.value._type or "" == "order") overridden
+            then sortProperties overridden
+            else overridden;
+        in sorted;
+
+        # Type-check the remaining definitions, and merge them
+        mergedValue = fold (def: res:
+          if type.check def.value then res
+          else throw "The option value `${showOption loc}' in `${def.file}' is not a ${type.name}.")
+          (type.merge loc defsFinal) defsFinal;
+    };
+
   /* Given a config set, expand mkMerge properties, and push down the
      other properties into the children.  The result is a list of
      config sets that do not have properties at top-level.  For