diff --git a/lib/src/fun/fun_factory.dart b/lib/src/fun/fun_factory.dart
index 8942f2d..2f8b2af 100644
--- a/lib/src/fun/fun_factory.dart
+++ b/lib/src/fun/fun_factory.dart
@@ -48,17 +48,37 @@ class FunFactory {
 
   Expression<T> _any1<T extends Object>(String name, Expression a0) {
     final f = _getFun1<T>(name);
-    final cast0 = cast(value: f is Fun1<T, Maybe>, logical: f is Fun1<T, bool>);
-    return a0.map(cast0).map(f.call);
+    if (f is Fun1<T, Maybe>) {
+      if (a0 is Expression<Maybe>) {
+        return a0.map(f.call);
+      }
+      if (a0 is Expression<SingularNodeList>) {
+        return a0.map((v) => v.asValue).map(f.call);
+      }
+    }
+    if (f is Fun1<T, bool>) {
+      if (a0 is Expression<bool>) {
+        return a0.map(f.call);
+      }
+      if (a0 is Expression<SingularNodeList>) {
+        return a0.map((v) => v.asLogical).map(f.call);
+      }
+    }
+    if (f is Fun1<T, NodeList>) {
+      if (a0 is Expression<NodeList>) {
+        return a0.map(f.call);
+      }
+    }
+    throw Exception('Fun arg mismatch');
   }
 
   Expression<T> _any2<T extends Object>(
       String name, Expression a0, Expression a1) {
     final f = _getFun2<T>(name);
-    final cast0 = cast(
+    final cast0 = cast(a0,
         value: f is Fun2<T, Maybe, Object>,
         logical: f is Fun2<T, bool, Object>);
-    final cast1 = cast(
+    final cast1 = cast(a1,
         value: f is Fun2<T, Object, Maybe>,
         logical: f is Fun2<T, Object, bool>);
     return a0.map(cast0).merge(a1.map(cast1), f.call);
@@ -76,9 +96,14 @@ class FunFactory {
     throw StateError('Function "$name" of 2 arguments is not found');
   }
 
-  static Object Function(Object) cast(
+  static Object Function(Object) cast(Expression arg,
       {required bool value, required bool logical}) {
-    if (value) return _value;
+    if (value) {
+      if (arg is! Expression<Maybe> && arg is! Expression<SingularNodeList>) {
+        throw Exception('Arg type mismatch');
+      }
+      return _value;
+    }
     if (logical) return _logical;
     return _nodes;
   }
diff --git a/lib/src/grammar/json_path.dart b/lib/src/grammar/json_path.dart
index 5bf3c89..e050331 100644
--- a/lib/src/grammar/json_path.dart
+++ b/lib/src/grammar/json_path.dart
@@ -62,7 +62,8 @@ class JsonPathGrammarDefinition
 
   Parser<Expression> _funArgument() => [
         literal,
-        _filterPath(),
+        ref0(_singularFilterPath),
+        ref0(_filterPath),
         ref0(_valueFunExpr),
         ref0(_logicalFunExpr),
         ref0(_nodesFunExpr),
@@ -113,7 +114,7 @@ class JsonPathGrammarDefinition
         ref0(_absPath),
       ].toChoiceParser();
 
-  Parser<Expression<NodeList>> _singularFilterPath() => [
+  Parser<Expression<SingularNodeList>> _singularFilterPath() => [
         ref0(_singularRelPath),
         ref0(_singularAbsPath),
       ].toChoiceParser();
@@ -156,12 +157,13 @@ class JsonPathGrammarDefinition
 
   Parser<Expression<SingularNodeList>> _singularAbsPath() =>
       _singularSegmentSequence()
-          .skip(before: char(r'$'))
+          .skip(before: char(r'$'), after: _segment().not())
           .map((expr) => Expression((node) => expr.call(node.root)));
 
   Parser<Expression<NodeList>> _relPath() =>
       _segmentSequence().skip(before: char('@'));
 
   Parser<Expression<SingularNodeList>> _singularRelPath() =>
-      _singularSegmentSequence().skip(before: char('@'));
+      _singularSegmentSequence()
+          .skip(before: char('@'), after: _segment().not());
 }
diff --git a/test/cases/extra/cases.json b/test/cases/extra/cases.json
index 6a3222d..754dec9 100644
--- a/test/cases/extra/cases.json
+++ b/test/cases/extra/cases.json
@@ -1,31 +1,31 @@
 {
   "tests": [
     {
-      "name": "reverse()",
+      "name": "reverse(@)",
       "selector" : "$[?reverse(@)=='cba']",
       "document" : ["abc", "cba"],
       "result": ["abc"]
     },
     {
-      "name": "reverse(reverse())",
+      "name": "reverse(reverse(@))",
       "selector" : "$[?reverse(reverse(@))=='cba']",
       "document" : ["abc", "cba"],
       "result": ["cba"]
     },
     {
-      "name": "reverse(reverse())",
+      "name": "count(siblings(@))",
       "selector" : "$..[?count(siblings(@)) == 1]",
       "document" : {"a":  {"b":  "x", "d":  "x"}},
       "result": ["x", "x"]
     },
     {
-      "name": "is_object",
+      "name": "is_object(@)",
       "selector" : "$[?is_object(@)]",
       "document" : [1, true, {}, [42], "foo", {"a": "b"}],
       "result": [{}, {"a": "b"}]
     },
     {
-      "name": "is_array",
+      "name": "is_array(@)",
       "selector" : "$[?is_array(@)]",
       "document" : [1, true, {}, [42], "foo", {"a": "b"}],
       "result": [[42]]
@@ -53,6 +53,74 @@
       "selector" : "$[?xor((@.b), (@.a))]",
       "document" : [{"a": 0}, {"a": 0, "b": 0}, {"b": 0}, {}],
       "result": [{"a": 0}, {"b": 0}]
+    },
+    {
+      "name": "functions, length, non-singular query arg",
+      "selector": "$[?length(@.*)<3]",
+      "invalid_selector": true
+    },
+    {
+      "name": "functions, length, arg is a function expression",
+      "selector": "$.values[?length(@.a)==length(value($..c))]",
+      "document": {
+        "c": "cd",
+        "values": [
+          {
+            "a": "ab"
+          },
+          {
+            "a": "d"
+          }
+        ]
+      },
+      "result": [
+        {
+          "a": "ab"
+        }
+      ]
+    }, {
+      "name": "functions, match, arg is a function expression",
+      "selector": "$.values[?match(@.a, value($..['regex']))]",
+      "document": {
+        "regex": "a.*",
+        "values": [
+          {
+            "a": "ab"
+          },
+          {
+            "a": "ba"
+          }
+        ]
+      },
+      "result": [
+        {
+          "a": "ab"
+        }
+      ]
+    },
+    {
+      "name": "functions, search, arg is a function expression",
+      "selector": "$.values[?search(@, value($..['regex']))]",
+      "document": {
+        "regex": "b.?b",
+        "values": [
+          "abc",
+          "bcd",
+          "bab",
+          "bba",
+          "bbab",
+          "b",
+          true,
+          [],
+          {}
+        ]
+      },
+      "result": [
+        "bab",
+        "bba",
+        "bbab"
+      ]
     }
+
   ]
 }
\ No newline at end of file
diff --git a/test/parser_test.dart b/test/parser_test.dart
index f92f8f3..a3d394e 100644
--- a/test/parser_test.dart
+++ b/test/parser_test.dart
@@ -29,3 +29,5 @@ void main() {
     });
   });
 }
+
+