feat: optional Variable.derive transform

2 files changed, 10 insertions, 3 deletions

diff --git a/gjs/src/variable.ts b/gjs/src/variable.ts
index 05e142f..d583ab1 100644
--- a/gjs/src/variable.ts
+++ b/gjs/src/variable.ts
@@ -195,13 +195,14 @@ class VariableWrapper<T> extends Function {
         return this as unknown as Variable<T>
     }
 
-    static derive<V,
+    static derive<
         const Deps extends Array<Variable<any> | Binding<any>>,
         Args extends {
             [K in keyof Deps]: Deps[K] extends Variable<infer T>
                 ? T : Deps[K] extends Binding<infer T> ? T : never
         },
-    >(deps: Deps, fn: (...args: Args) => V) {
+        V = Args,
+    >(deps: Deps, fn: (...args: Args) => V = (...args) => args as unknown as V) {
         const update = () => fn(...deps.map(d => d.get()) as Args)
         const derived = new Variable(update())
         const unsubs = deps.map(dep => dep.subscribe(() => derived.set(update())))
diff --git a/lua/astal/variable.lua b/lua/astal/variable.lua
index c7c2ec9..1e894b5 100644
--- a/lua/astal/variable.lua
+++ b/lua/astal/variable.lua
@@ -223,9 +223,15 @@ function Variable:observe(object, sigOrFn, callback)
 end
 
 ---@param deps Variable | (Binding | Variable)[]
----@param transform fun(...): any
+---@param transform? fun(...): any
 ---@return Variable
 function Variable.derive(deps, transform)
+    if type(transform) == "nil" then
+        transform = function(...)
+            return { ... }
+        end
+    end
+
     if getmetatable(deps) == Variable then
         local var = Variable.new(transform(deps:get()))
         deps:subscribe(function(v)