diff --git a/src/Updux.ts b/src/Updux.ts
index d275700..abb2155 100644
--- a/src/Updux.ts
+++ b/src/Updux.ts
@@ -41,11 +41,13 @@ type ResolveActions<
         : never;
 };
 
-type Mutation<A extends ActionCreator<any> = ActionCreator<any>, S = any> = (
-    state: S,
+export type Mutation<
+    A extends ActionCreator<any> = ActionCreator<any>,
+    S = any,
+> = (
     payload: ReturnType<A>['payload'],
     action: ReturnType<A>,
-) => S | void;
+) => (state: S) => S | void;
 
 export default class Updux<
     T_LocalState = Record<any, any>,
diff --git a/src/reducer.test.todo b/src/reducer.test.todo
deleted file mode 100644
index 2b5ff18..0000000
--- a/src/reducer.test.todo
+++ /dev/null
@@ -1,19 +0,0 @@
-import { test, expect } from 'vitest';
-
-import { Updux } from './Updux.js';
-
-test('basic reducer', () => {
-    const dux = new Updux({ initial: { a: 3 } });
-
-    expect(dux.reducer).toBeTypeOf('function');
-
-    expect(dux.reducer({ a: 1 }, { type: 'foo' })).toMatchObject({ a: 1 }); // noop
-});
-
-test('basic upreducer', () => {
-    const dux = new Updux({ initial: { a: 3 } });
-
-    expect(dux.upreducer).toBeTypeOf('function');
-
-    expect(dux.upreducer({ type: 'foo' })({ a: 1 })).toMatchObject({ a: 1 }); // noop
-});
diff --git a/src/reducer.test.ts b/src/reducer.test.ts
new file mode 100644
index 0000000..9ce0ba8
--- /dev/null
+++ b/src/reducer.test.ts
@@ -0,0 +1,31 @@
+import { test, expect } from 'vitest';
+
+import { buildReducer } from './reducer.js';
+import Updux from './Updux.js';
+
+test('buildReducer, initial state', () => {
+    const reducer = buildReducer({ a: 1 });
+
+    expect(reducer(undefined, { type: 'foo' })).toEqual({ a: 1 });
+});
+
+test('buildReducer, mutation', () => {
+    const reducer = buildReducer(1, [
+        {
+            matcher: ({ type }) => type === 'inc',
+            mutation: () => (state) => state + 1,
+            terminal: false,
+        },
+    ]);
+
+    expect(reducer(undefined, { type: 'foo' })).toEqual(1);
+    expect(reducer(undefined, { type: 'inc' })).toEqual(2);
+});
+
+test.todo('basic reducer', () => {
+    const dux = new Updux({ initial: { a: 3 } });
+
+    expect(dux.reducer).toBeTypeOf('function');
+
+    expect(dux.reducer({ a: 1 }, { type: 'foo' })).toMatchObject({ a: 1 }); // noop
+});
diff --git a/src/reducer.ts b/src/reducer.ts
new file mode 100644
index 0000000..0d87941
--- /dev/null
+++ b/src/reducer.ts
@@ -0,0 +1,79 @@
+import { Action, ActionCreator, createAction } from '@reduxjs/toolkit';
+import { BaseActionCreator } from '@reduxjs/toolkit/dist/createAction.js';
+import * as R from 'remeda';
+import { Dux } from './types.js';
+import { Mutation } from './Updux.js';
+
+type MutationCase = {
+    matcher: (action: Action) => boolean;
+    mutation: Mutation;
+    terminal: boolean;
+};
+
+export function buildReducer(
+    initialState: any,
+    mutations: MutationCase[] = [],
+    subduxes: Record<string, Dux> = {},
+) {
+    // const subReducers =
+    //         ? R.mapValues(subduxes, R.prop('reducer'));
+
+    // TODO matcherMutation
+    // TODO defaultMutation
+
+    const reducer = (state = initialState, action: Action) => {
+        if (!action?.type)
+            throw new Error('upreducer called with a bad action');
+
+        let terminal = false;
+        let didSomething = false;
+
+        const foo = createAction('foo');
+
+        const localMutation = mutations.find(({ matcher }) => matcher(action));
+
+        if (localMutation) {
+            didSomething = true;
+            if (localMutation.terminal) terminal = true;
+
+            // TODO wrap mutations in immer
+            state = localMutation.mutation(
+                (action as any).payload,
+                action,
+            )(state);
+        }
+
+        // TODO defaultMutation
+
+        return state;
+    };
+
+    return reducer;
+
+    /*
+            if (subReducers) {
+                if (subduxes['*']) {
+                    newState = u.updateIn(
+                        '*',
+                        subduxes['*'].upreducer(action),
+                        newState,
+                    );
+                } else {
+                    const update = mapValues(subReducers, (upReducer) =>
+                        upReducer(action),
+                    );
+    
+                    newState = u(update, newState);
+                }
+            }
+    
+            const a = mutations[action.type] || mutations['+'];
+    
+            if (!a) return newState;
+    
+            return a(action.payload, action)(newState);
+        };
+    
+        return wrapper ? wrapper(upreducer) : upreducer;
+            */
+}