diff --git a/README.md b/README.md
index 1c93be5f..5e052291 100644
--- a/README.md
+++ b/README.md
@@ -52,7 +52,7 @@ Expands expressions in a configuration object
 | Param | Type | Description |
 | --- | --- | --- |
 | config | <code>object</code> |  |
-| [options] | <code>object</code> | constants object holding additional constants |
+| [options] | <code>object</code> | constants object holding additional constants    functions object holding additional function |
 
 
 * <a name="module_config-expander..functions.split.apply"></a>
diff --git a/src/expander.js b/src/expander.js
index ee3348cb..8e6f7816 100644
--- a/src/expander.js
+++ b/src/expander.js
@@ -17,11 +17,22 @@ import {
 }
 from './grammar';
 
+import {
+	functions
+}
+from './functions';
+
+import {
+	createValue
+}
+from './util';
+
 /**
  * Expands expressions in a configuration object
  * @param {object} config
  * @param {object} [options]
  *    constants object holding additional constants
+ *    functions object holding additional function
  * @returns {Promise} expanded configuration
  */
 export function expand(config, options = {}) {
@@ -30,13 +41,14 @@ export function expand(config, options = {}) {
 			constants: Object.assign({
 				basedir: '/',
 				os: os
-			}, options.constants)
+			}, options.constants),
+			functions: Object.assign( {}, functions, options.functions)
 		};
 
 		const parser = new ConfigParser();
 
 		const ee = createContext({
-			evaluate: (expression, _context, path) => {
+			evaluate: (expression, _unusedContext, path) => {
 				context.path = path;
 				const ast = parser.parse(expression, context);
 				return ast.value;
@@ -48,3 +60,5 @@ export function expand(config, options = {}) {
 		return Promise.reject(e);
 	}
 }
+
+export { createValue };
diff --git a/src/grammar.js b/src/grammar.js
index eb6ecd39..dfe07b8c 100644
--- a/src/grammar.js
+++ b/src/grammar.js
@@ -11,11 +11,6 @@ import {
 }
 from './util';
 
-import {
-	functions
-}
-from './functions';
-
 class AST {
 	get value() {
 		return undefined;
@@ -185,7 +180,7 @@ const grammar = {
 
 					grammar.advance(')');
 
-					const f = functions[left.value];
+					const f = grammar.context.functions[left.value];
 					if (f) {
 						//console.log(`${f.arguments} <> ${args.map(a => a.type)}`);
 						return new FCall(f, grammar.context, args);
diff --git a/tests/simple_test.js b/tests/simple_test.js
index d23f1ac0..1826a237 100644
--- a/tests/simple_test.js
+++ b/tests/simple_test.js
@@ -10,7 +10,7 @@ const chai = require('chai'),
   expect = chai.expect,
   should = chai.should();
 
-const expand = require('../dist/expander').expand;
+const {expand, createValue} = require('../dist/expander');
 
 describe('expander', () => {
 
@@ -145,6 +145,15 @@ describe('expander', () => {
       r, 'secret')));
   });
 
+  describe('user defined functions', () => {
+    it('can call', () => expand(
+      "${myFunction()}", {
+        functions: {
+          myFunction: { apply: (context, args) => { return createValue(77); }}
+        }
+      }).then(r => assert.equal(r, 77)));
+  });
+
   describe('promise function args', () => {
     it('one promise arg', () => expand(
       "${substring(string(document('fixtures/short.txt')),0,4)}", {