diff --git a/src/buildMutations/index.js b/src/buildMutations/index.js
deleted file mode 100644
index f7dab81..0000000
--- a/src/buildMutations/index.js
+++ /dev/null
@@ -1,53 +0,0 @@
-import fp from 'lodash/fp';
-import u from 'updeep';
-
-const composeMutations = (mutations) =>
-    mutations.reduce( (m1,m2) =>
-        (payload=null,action={}) => state => m2(payload,action)(
-            m1(payload,action)(state) ));
-
-export default function buildMutations(mutations = {}, subduxes= {}) {
-  // we have to differentiate the subduxes with '*' than those
-  // without, as the root '*' is not the same as any sub-'*'
-
-    const actions = fp.uniq( Object.keys(mutations).concat(
-        ...Object.values( subduxes ).map( ({mutations = {}}) => Object.keys(mutations) )
-    ) );
-
-    let mergedMutations = {};
-
-    let [ globby, nonGlobby ] = fp.partition(
-        ([_,{mutations={}}]) => mutations['*'],
-        Object.entries(subduxes)
-    );
-
-    globby =
-        fp.flow([
-            fp.fromPairs,
-            fp.mapValues(
-        ({reducer}) => (_,action={}) => state =>
-            reducer(state,action) ),
-        ])(globby);
-
-    const globbyMutation = (payload,action) => u(
-        fp.mapValues( (mut) => mut(payload,action) )(globby)
-    );
-
-    actions.forEach( action => {
-        mergedMutations[action] = [ globbyMutation ]
-    });
-
-    nonGlobby.forEach( ([slice, {mutations={},reducer={}}]) => {
-        Object.entries(mutations).forEach(([type,mutation]) => {
-            const localized = (payload=null,action={}) => u.updateIn( slice )( (mutation)(payload,action) );
-
-            mergedMutations[type].push(localized);
-        })
-    });
-
-    Object.entries(mutations).forEach(([type,mutation]) => {
-            mergedMutations[type].push(mutation);
-    });
-
-    return fp.mapValues( composeMutations )(mergedMutations);
-}
diff --git a/src/buildMutations/index.ts b/src/buildMutations/index.ts
new file mode 100644
index 0000000..472a1a0
--- /dev/null
+++ b/src/buildMutations/index.ts
@@ -0,0 +1,66 @@
+import fp from 'lodash/fp';
+import u from 'updeep';
+import {Mutation, Action, Dictionary} from '../types';
+
+const composeMutations = (mutations: Mutation[]) =>
+  mutations.reduce((m1, m2) => (payload: any = null, action: Action) => state =>
+    m2(payload, action)(m1(payload, action)(state)),
+  );
+
+type SubMutations = {
+    [ slice: string ]: Dictionary<Mutation>
+}
+
+function buildMutations(
+    mutations :Dictionary<Mutation> = {},
+    subduxes = {}
+) {
+  // we have to differentiate the subduxes with '*' than those
+  // without, as the root '*' is not the same as any sub-'*'
+
+  const actions = fp.uniq(
+    Object.keys(mutations).concat(
+      ...Object.values(subduxes).map(({mutations = {}}:any) =>
+        Object.keys(mutations),
+      ),
+    ),
+  );
+
+  let mergedMutations :Dictionary<Mutation[]> = {};
+
+  let [globby, nonGlobby] = fp.partition(
+    ([_, {mutations = {}}]:any) => mutations['*'],
+    Object.entries(subduxes),
+  );
+
+  globby = fp.flow([
+    fp.fromPairs,
+    fp.mapValues(({reducer}) => (_:any, action :Action) => ( state: any ) =>
+      reducer(state, action),
+    ),
+  ])(globby);
+
+  const globbyMutation = (payload:any, action:Action) =>
+    u(fp.mapValues((mut:any) => mut(payload, action))(globby));
+
+  actions.forEach(action => {
+    mergedMutations[action] = [globbyMutation];
+  });
+
+  nonGlobby.forEach(([slice, {mutations = {}, reducer = {}}]:any[]) => {
+    Object.entries(mutations).forEach(([type, mutation]) => {
+      const localized = (payload = null, action :Action) =>
+        u.updateIn(slice)((mutation as Mutation)(payload, action));
+
+      mergedMutations[type].push(localized);
+    });
+  });
+
+  Object.entries(mutations).forEach(([type, mutation]) => {
+    mergedMutations[type].push(mutation);
+  });
+
+  return fp.mapValues(composeMutations)(mergedMutations);
+}
+
+export default buildMutations;
diff --git a/src/types.ts b/src/types.ts
index 78701e6..5bacf08 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -7,7 +7,7 @@ export type Action = {
 
 export type Dictionary<T> = { [key: string]: T };
 
-export type Mutation<S> = (payload: any, action: Action) => (state: S) => S ;
+export type Mutation<S=any> = (payload: any, action: Action) => (state: S) => S ;
 
 export type ActionCreator = (...args: any[] ) => Action;