add vide.mutate()

src/init.luau

@@ -16,6 +16,7 @@ local cleanup = require(script.cleanup)
 local untrack = require(script.untrack)
 local read = require(script.read)
 local batch = require(script.batch)
+local mutate = require(script.mutate)
 local switch = require(script.switch)
 local show = require(script.show)
 local indexes, values = require(script.maps)()
@@ -63,6 +64,7 @@ local vide = {
     untrack = untrack,
     read = read,
     batch = batch,
+    mutate = mutate,
 
     -- animations
     spring = spring,

src/mutate.luau

@@ -0,0 +1,15 @@
+if not game then script = require "test/relative-string" end
+
+local untrack = require(script.Parent.untrack)
+
+local function mutate<T>(source: () -> T, mutator: (T) -> any?)
+    local value = untrack(source)
+    local packed = table.pack(mutator(value))
+    if packed.n > 0 then
+        return source(unpack(packed, 1, packed.n))
+    end
+
+    source(value)
+end
+
+return mutate