diff --git a/package.json b/package.json
index 1a24cd2..450abe0 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "json-expression-eval",
-  "version": "3.2.0",
+  "version": "3.3.0",
   "description": "evaluate a json described boolean expression using dynamic functions",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
diff --git a/src/example/example.ts b/src/example/example.ts
index c99f687..9106836 100644
--- a/src/example/example.ts
+++ b/src/example/example.ts
@@ -102,13 +102,14 @@ run(expression, context);
 
 const evaluator = getEvaluator(expression);
 
-const expressionParts: GetExpressionParts<typeof evaluator> = {
+const expressionParts: GetExpressionParts<typeof evaluator, { description: string }> = {
     nested: {
         value: {
             isArray: false,
             isFunction: false,
             propertyPath: 'nested.value',
             type: 'number',
+            description: 'my desc',
         },
         nested2: {
             value: {
@@ -116,6 +117,7 @@ const expressionParts: GetExpressionParts<typeof evaluator> = {
                 isFunction: false,
                 propertyPath: 'nested.nested2.value',
                 type: 'number',
+                description: 'my desc',
             },
         },
     },
@@ -124,23 +126,27 @@ const expressionParts: GetExpressionParts<typeof evaluator> = {
         isFunction: false,
         propertyPath: 'times',
         type: 'number',
+        description: 'my desc',
     },
     userId: {
         isArray: false,
         isFunction: false,
         propertyPath: 'userId',
         type: 'string',
+        description: 'my desc',
     },
     maxCount: {
         isArray: false,
         isFunction: true,
         propertyPath: 'maxCount',
         type: 'number',
+        description: 'my desc',
     },
     user: {
         isArray: false,
         isFunction: true,
         propertyPath: 'user',
         type: 'string',
+        description: 'my desc',
     },
 } as const;
diff --git a/src/expressionParts.ts b/src/expressionParts.ts
index 2569dd8..ad235af 100644
--- a/src/expressionParts.ts
+++ b/src/expressionParts.ts
@@ -12,8 +12,8 @@ interface ExpressionFunctionPart<C extends Context, F extends FunctionsTable<C>,
     isFunction: true;
 }
 
-type ExpressionFunctionParts<C extends Context, F extends FunctionsTable<C>> = {
-    [K in keyof F]: ExpressionFunctionPart<C, F, K>
+type ExpressionFunctionParts<C extends Context, F extends FunctionsTable<C>, Extra extends object> = {
+    [K in keyof F]: ExpressionFunctionPart<C, F, K> & Extra;
 }
 
 interface ExpressionContextPart<C extends Context, K extends keyof C, P extends List.List<string>> {
@@ -25,15 +25,15 @@ interface ExpressionContextPart<C extends Context, K extends keyof C, P extends
 
 type Primitive = string | number | boolean;
 
-type _ExpressionContextParts<C, P extends List.List<string>> = {
+type _ExpressionContextParts<C, P extends List.List<string>, Extra extends object> = {
     [k in U.Select<keyof C, string>]:
-    C[k] extends Primitive ? ExpressionContextPart<C, k, [...P, k]>
+    C[k] extends Primitive ? ExpressionContextPart<C, k, [...P, k]> & Extra
         : C[k] extends Array<any> ? never
-        : _ExpressionContextParts<C[k], [...P, k]>;
+        : _ExpressionContextParts<C[k], [...P, k], Extra>;
 }
 
-type ExpressionContextParts<C extends Context> = _ExpressionContextParts<C, []>;
+type ExpressionContextParts<C extends Context, Extra extends object> = _ExpressionContextParts<C, [], Extra>;
 
-export type ExpressionParts<C extends Context, F extends FunctionsTable<C>> =
-    ExpressionFunctionParts<C, F>
-    & ExpressionContextParts<C>;
+export type ExpressionParts<C extends Context, F extends FunctionsTable<C>, Extra extends object = {}> =
+    ExpressionFunctionParts<C, F, Extra>
+    & ExpressionContextParts<C, Extra>;
diff --git a/src/index.ts b/src/index.ts
index 40b0ef3..6c1abb2 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -23,5 +23,5 @@ export class ExpressionEval<C extends Context, F extends FunctionsTable<C>> {
 
 }
 
-export type GetExpressionParts<E extends ExpressionEval<any, any>> =
-    E extends ExpressionEval<infer C, infer F> ? ExpressionParts<C, F> : never;
+export type GetExpressionParts<E extends ExpressionEval<any, any>, Extra extends object = {}> =
+    E extends ExpressionEval<infer C, infer F> ? ExpressionParts<C, F, Extra> : never;
diff --git a/src/test/expressionParts.test-d.ts b/src/test/expressionParts.test-d.ts
index ed88619..081e401 100644
--- a/src/test/expressionParts.test-d.ts
+++ b/src/test/expressionParts.test-d.ts
@@ -97,6 +97,94 @@ const expected = {
     },
 } as const;
 
+type ResultExtended =
+    O.Readonly<ExpressionParts<ExpressionContext, ExpressionFunction, { description: string }>, Any.Key, 'deep'>;
+
+const expectedExtended = {
+    nested: {
+        value: {
+            isArray: false,
+            isFunction: false,
+            propertyPath: 'nested.value',
+            type: 'number',
+            description: 'my desc' as string,
+        },
+        nested2: {
+            value: {
+                isArray: false,
+                isFunction: false,
+                propertyPath: 'nested.nested2.value',
+                type: 'string',
+                description: 'my desc' as string,
+            },
+        },
+    },
+    str: {
+        isArray: false,
+        isFunction: false,
+        propertyPath: 'str',
+        type: 'string',
+        description: 'my desc' as string,
+    },
+    strFn: {
+        isArray: false,
+        isFunction: true,
+        propertyPath: 'strFn',
+        type: 'string',
+        description: 'my desc' as string,
+    },
+    strArrFn: {
+        isArray: true,
+        isFunction: true,
+        propertyPath: 'strArrFn',
+        type: 'string',
+        description: 'my desc' as string,
+    },
+    num: {
+        isArray: false,
+        isFunction: false,
+        propertyPath: 'num',
+        type: 'number',
+        description: 'my desc' as string,
+    },
+    numFn: {
+        isArray: false,
+        isFunction: true,
+        propertyPath: 'numFn',
+        type: 'number',
+        description: 'my desc' as string,
+    },
+    numArrFn: {
+        isArray: true,
+        isFunction: true,
+        propertyPath: 'numArrFn',
+        type: 'number',
+        description: 'my desc' as string,
+    },
+    bool: {
+        isArray: false,
+        isFunction: false,
+        propertyPath: 'bool',
+        type: 'boolean',
+        description: 'my desc' as string,
+    },
+    boolFn: {
+        isArray: false,
+        isFunction: true,
+        propertyPath: 'boolFn',
+        type: 'boolean',
+        description: 'my desc' as string,
+    },
+    boolArrFn: {
+        isArray: true,
+        isFunction: true,
+        propertyPath: 'boolArrFn',
+        type: 'boolean',
+        description: 'my desc' as string,
+    },
+} as const;
+
 Test.checks([
     Test.check<Result, typeof expected, Test.Pass>(),
+    Test.check<ResultExtended, typeof expectedExtended, Test.Pass>(),
 ]);