lib: support gvariant maybe type

Fixes #1397

doc/writing-modules.adoc

@@ -168,13 +168,20 @@ Builds a GVariant array containing the given list of elements, where each elemen
 - `hm.gvariant.type.uint64`
 - `hm.gvariant.type.double`
 - `hm.gvariant.type.arrayOf type`
+- `hm.gvariant.type.maybeOf type`
 - `hm.gvariant.type.tupleOf types`
 --
 +
-where `type` and `types` are themselve a type and list of types, respectively.
+where `type` and `types` are themselves a type and list of types, respectively.
 +
 `hm.gvariant.mkEmptyArray type`:::
 An alias of `hm.gvariant.mkArray type []`.
 +
+`hm.gvariant.mkNothing type`:::
+Builds a GVariant maybe value whose (non-existent) element is of the given type. The `type` value is constructed as described for the `mkArray` function above.
++
+`hm.gvariant.mkJust element`:::
+Builds a GVariant maybe value containing the given GVariant element.
++
 `hm.gvariant.mkTuple elements`:::
 Builds a GVariant tuple containing the given list of elements, where each element is a GVariant value.

modules/lib/gvariant.nix

@@ -18,6 +18,7 @@ let
 
   type = {
     arrayOf = t: "a${t}";
+    maybeOf = t: "m${t}";
     tupleOf = ts: "(${concatStrings ts})";
     string = "s";
     boolean = "b";
@@ -57,11 +58,21 @@ let
     else
       "";
 
+  mkMaybe = elemType: elem:
+    mkPrimitive (type.maybeOf elemType) elem // {
+      __toString = self:
+        if self.value == null then
+          "@${self.type} nothing"
+        else
+          "just ${toString self.value}";
+    };
+
 in rec {
 
   inherit type typeOf;
 
   isArray = hasPrefix "a";
+  isMaybe = hasPrefix "m";
   isTuple = hasPrefix "(";
 
   # Returns the GVariant value that most closely matches the given Nix
@@ -92,6 +103,10 @@ in rec {
 
   mkEmptyArray = elemType: mkArray elemType [ ];
 
+  mkNothing = elemType: mkMaybe elemType null;
+
+  mkJust = elem: let gvarElem = mkValue elem; in mkMaybe gvarElem.type gvarElem;
+
   mkTuple = elems:
     let
       gvarElems = map mkValue elems;

modules/lib/types.nix

@@ -77,6 +77,8 @@ in rec {
         (map (d: d // { value = d.value.value; }) vdefs)
       else if gvar.isTuple sharedDefType && allChecked then
         mergeOneOption loc defs
+      else if gvar.isMaybe 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

tests/lib/types/gvariant-merge.nix

@@ -28,6 +28,9 @@ in {
         { string = "foo"; }
 
         { tuple = mkTuple [ 1 [ "foo" ] ]; }
+
+        { maybe1 = mkNothing type.string; }
+        { maybe2 = mkJust (mkUint32 4); }
       ];
 
     home.file."result.txt".text = let
@@ -46,6 +49,8 @@ in {
             float = 3.140000
             int = 42
             list = @as ['one','two']
+            maybe1 = @ms nothing
+            maybe2 = just @u 4
             string = 'foo'
             tuple = @(ias) (1,@as ['foo'])
           ''