lib: add GVariant datatype and functions

modules/lib/types.nix

@@ -1,4 +1,7 @@
-{ lib, dag ? import ./dag.nix { inherit lib; } }:
+{ lib
+, dag ? import ./dag.nix { inherit lib; }
+, gvariant ? import ./gvariant.nix { inherit lib; }
+}:
 
 with lib;
 
@@ -6,9 +9,13 @@ let
 
   typesDag = import ./types-dag.nix { inherit dag lib; };
 
+  # Needed since the type is called gvariant and its merge attribute
+  # must refer back to the type.
+  gvar = gvariant;
+
 in
 
-{
+rec {
 
   inherit (typesDag) dagOf listOrDagOf;
 
@@ -56,4 +63,35 @@ in
     };
   };
 
+  gvariant = mkOptionType rec {
+    name = "gvariant";
+    description = "GVariant value";
+    check = v: gvar.mkValue v != null;
+    merge = loc: defs:
+      let
+        vdefs = map (d: d // { value = gvar.mkValue d.value; }) defs;
+        vals = map (d: d.value) vdefs;
+        defTypes = map (x: x.type) vals;
+        sameOrNull = x: y: if x == y then y else null;
+        # A bit naive to just check the first entry…
+        sharedDefType = foldl' sameOrNull (head defTypes) defTypes;
+        allChecked = all (x: check x) vals;
+      in
+        if sharedDefType == null then
+          throw ("Cannot merge definitions of `${showOption loc}' with"
+                 + " mismatched GVariant types given in"
+                 + " ${showFiles (getFiles defs)}.")
+        else if gvar.isArray sharedDefType && allChecked then
+          (types.listOf gvariant).merge
+            loc (map (d: d // { value = d.value.value; } ) vdefs)
+        else if gvar.isTuple sharedDefType && allChecked then
+          mergeOneOption loc defs
+        else if gvar.type.string == sharedDefType && allChecked then
+          types.str.merge loc defs
+        else if gvar.type.double == sharedDefType && allChecked then
+          types.float.merge loc defs
+        else
+          mergeDefaultOption loc defs;
+  };
+
 }