diff --git a/lib/generic.ts b/lib/generic.ts
index 873897a1..4b73ad94 100644
--- a/lib/generic.ts
+++ b/lib/generic.ts
@@ -4,6 +4,7 @@ export {
   GenericProvable,
   GenericProvablePure,
   GenericProvableExtended,
+  GenericProvableExtendedPure,
   GenericField,
   GenericBool,
   GenericHashInput,
@@ -29,13 +30,22 @@ interface GenericProvablePure<T, Field> extends GenericProvable<T, Field> {
   check: (x: T) => void;
 }
 
-type GenericProvableExtended<T, TJson, Field> = GenericProvable<T, Field> & {
+type ProvableExtension<T, TJson, Field> = {
   toInput: (x: T) => { fields?: Field[]; packed?: [Field, number][] };
   toJSON: (x: T) => TJson;
   fromJSON: (x: TJson) => T;
   emptyValue?: () => T;
 };
 
+type GenericProvableExtended<T, TJson, Field> = GenericProvable<T, Field> &
+  ProvableExtension<T, TJson, Field>;
+
+type GenericProvableExtendedPure<T, TJson, Field> = GenericProvablePure<
+  T,
+  Field
+> &
+  ProvableExtension<T, TJson, Field>;
+
 type GenericField<Field> = ((value: number | string | bigint) => Field) &
   GenericProvableExtended<Field, string, Field> &
   Binable<Field> & { sizeInBytes(): number };
diff --git a/lib/provable-generic.ts b/lib/provable-generic.ts
index dbb80d0b..5d5fbc5e 100644
--- a/lib/provable-generic.ts
+++ b/lib/provable-generic.ts
@@ -3,6 +3,7 @@ import {
   GenericProvable,
   GenericProvablePure,
   GenericProvableExtended,
+  GenericProvableExtendedPure,
 } from './generic.js';
 
 export {
@@ -180,7 +181,7 @@ function createProvable<Field>(): ProvableConstructor<Field> {
       let keys = isToplevel ? objectKeys : Object.keys(typeObj);
       let values = fromJSON(
         keys.map((k) => typeObj[k]),
-        json
+        keys.map((k) => json[k])
       );
       return Object.fromEntries(keys.map((k, i) => [k, values[i]]));
     }
@@ -353,12 +354,6 @@ type IsPureBase<A, Field> = A extends GenericProvablePure<any, Field>
     }[keyof A]
   : false;
 
-type GenericProvableExtendedPure<T, TJson, Field> = GenericProvablePure<
-  T,
-  Field
-> &
-  GenericProvableExtended<T, TJson, Field>;
-
 type InferredProvable<A, Field> = IsPure<A, Field> extends true
   ? GenericProvableExtendedPure<InferProvable<A, Field>, InferJson<A>, Field>
   : GenericProvableExtended<InferProvable<A, Field>, InferJson<A>, Field>;
diff --git a/lib/provable-snarky.ts b/lib/provable-snarky.ts
index 185b2ee4..73e6863a 100644
--- a/lib/provable-snarky.ts
+++ b/lib/provable-snarky.ts
@@ -7,10 +7,10 @@ import {
   InferJson,
   InferredProvable as GenericInferredProvable,
   IsPure as GenericIsPure,
+  createHashInput,
 } from './provable-generic.js';
 import { Tuple } from '../../lib/util/types.js';
-
-// TODO make this whole file reuse ./provable-generic.ts
+import { GenericHashInput } from './generic.js';
 
 // external API
 export { ProvableExtended, provable, provablePure, provableTuple };
@@ -29,6 +29,7 @@ type ProvableExtension<T, TJson = any> = {
   toInput: (x: T) => { fields?: Field[]; packed?: [Field, number][] };
   toJSON: (x: T) => TJson;
   fromJSON: (x: TJson) => T;
+  emptyValue?: () => T;
 };
 type ProvableExtended<T, TJson = any> = Provable<T> &
   ProvableExtension<T, TJson>;
@@ -39,18 +40,8 @@ type InferProvable<T> = GenericInferProvable<T, Field>;
 type InferredProvable<T> = GenericInferredProvable<T, Field>;
 type IsPure<T> = GenericIsPure<T, Field>;
 
-type HashInput = { fields?: Field[]; packed?: [Field, number][] };
-const HashInput = {
-  get empty() {
-    return {};
-  },
-  append(input1: HashInput, input2: HashInput): HashInput {
-    return {
-      fields: (input1.fields ?? []).concat(input2.fields ?? []),
-      packed: (input1.packed ?? []).concat(input2.packed ?? []),
-    };
-  },
-};
+type HashInput = GenericHashInput<Field>;
+const HashInput = createHashInput<Field>();
 
 const provable = createProvable<Field>();