diff --git a/examples/parsersample.ts b/examples/parsersample.ts
index f25d66ed3..2cc832489 100644
--- a/examples/parsersample.ts
+++ b/examples/parsersample.ts
@@ -16,6 +16,13 @@ export class SampleParser implements StyleParser {
           info: 'Blur is only partially supported'
         }
       }
+    },
+    Function: {
+      acos: 'none',
+      strAbbreviate: {
+        support: 'partial',
+        info: 'Better use strSubstring instead.'
+      }
     }
   };
 
diff --git a/functions.ts b/functions.ts
index 90c1575b7..5a24eb499 100644
--- a/functions.ts
+++ b/functions.ts
@@ -120,9 +120,9 @@ export interface Fatan2 extends FunctionCall<number> {
 export interface Fbetween extends FunctionCall<boolean> {
   name: 'between';
   args: [
-    Expression<string>,
-    Expression<string>,
-    Expression<string>
+    Expression<number>,
+    Expression<number>,
+    Expression<number>
   ];
 };
 
diff --git a/style.ts b/style.ts
index 6d4d10381..2ffb9eabb 100644
--- a/style.ts
+++ b/style.ts
@@ -1,5 +1,6 @@
 import {
   GeoStylerBooleanFunction,
+  GeoStylerFunction,
   GeoStylerNumberFunction,
   GeoStylerStringFunction
 } from './functions';
@@ -80,12 +81,13 @@ export type RangeFilter = [
 ];
 
 /**
- * A ComparisonFilter compares a value of an object (by key) with an expected
- * value.
+ * A ComparisonFilter compares two values.
+ * If the fist argument is a GeoStylerFunction it will be evaluated it.
+ * If it is a string it will be treated as key of an object.
  */
 export type ComparisonFilter = [
   ComparisonOperator,
-  Expression<string>,
+  Expression<string | number | boolean>,
   Expression<string | number | boolean>
 ] | RangeFilter;
 
@@ -733,6 +735,9 @@ export interface UnsupportedProperties {
       [key in keyof Required<RasterSymbolizer>]?: SupportDef
     };
   };
+  Function?: SupportDef | {
+    [key in GeoStylerFunction['name']]?: SupportDef;
+  };
 }
 
 /**