refactor!: rename derive, map

src/mapped.luau

@@ -0,0 +1,53 @@
+local atom = require(script.Parent.atom)
+local effect = require(script.Parent.effect)
+local peek = require(script.Parent.peek)
+local types = require(script.Parent.types)
+type Atom<T> = types.Atom<T>
+type Molecule<T> = types.Molecule<T>
+
+type Map =
+	(<K0, V0, K1, V1>(fn: Molecule<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1)) -> Molecule<{ [K1]: V1 }>)
+	& (<K0, V0, V1>(fn: Molecule<{ [K0]: V0 }>, mapper: (V0, K0) -> V1?) -> Molecule<{ [K0]: V1 }>)
+	& (<K0, V0, K1, V1>(fn: Molecule<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1?)) -> Molecule<{ [K1]: V1 }>)
+
+local function mapped<K0, V0, K1, V1>(molecule: Molecule<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1?)): Molecule<{ [K1]: V1 }>
+	local mappedAtom = atom({})
+	local mappedAtomRef = setmetatable({ current = mappedAtom }, { __mode = "v" })
+	local prevMappedItems: { [K1]: V1 } = {}
+	local unsubscribe
+
+	unsubscribe = effect(function()
+		if not mappedAtomRef.current then
+			return unsubscribe()
+		end
+
+		local items = molecule()
+		local mappedItems = table.clone(peek(mappedAtomRef.current))
+		local mappedKeys = {}
+
+		for key, item in items do
+			local newItem, newKey = mapper(item, key)
+			if newKey == nil then
+				newKey = key :: any
+			end
+			if mappedItems[newKey :: K1] ~= newItem then
+				mappedItems[newKey :: K1] = newItem :: V1
+			else
+				mappedKeys[newKey] = key
+			end
+		end
+
+		for key in prevMappedItems do
+			if mappedKeys[key] == nil and mappedItems[key] == prevMappedItems[key] then
+				mappedItems[key] = nil
+			end
+		end
+
+		prevMappedItems = mappedItems
+		mappedAtom(mappedItems)
+	end)
+
+	return mappedAtom
+end
+
+return mapped :: Map