@@ -3,13 +3,21 @@ package ast
 import (
 "reflect"
 
+".com/expr-lang/expr/checker/nature"
 ".com/expr-lang/expr/file"
 )
 
+var (
+anyType = reflect.TypeOf(new(any)).Elem()
+)
+
 // Node represents items of abstract syntax tree.
 type Node interface {
 Location() file.Location
 SetLocation(file.Location)
+Nature() nature.Nature
+SetNature(nature.Nature)
+Kind() reflect.Kind
 Type() reflect.Type
 SetType(reflect.Type)
 String() string
@@ -25,8 +33,8 @@ func (node *Node, newNode Node) {
 
 // base is a base struct for all nodes.
 type base struct {
-loc file.Location
-nodeType reflect.Type
+loc file.Location
+nature nature.Nature
 }
 
 // Location returns the location of the node in the source code.
@@ -39,14 +47,36 @@ func (n *base) SetLocation(loc file.Location) {
 n.loc = loc
 }
 
+// Nature returns the nature of the node.
+func (n *base) Nature() nature.Nature {
+return n.nature
+}
+
+// SetNature sets the nature of the node.
+func (n *base) SetNature(nature nature.Nature) {
+n.nature = nature
+}
+
+// Kind returns the kind of the node.
+// If the type is nil (meaning unknown) then it returns reflect.Interface.
+func (n *base) Kind() reflect.Kind {
+if n.nature.Type == nil {
+return reflect.Interface
+}
+return n.nature.Type.Kind()
+}
+
 // Type returns the type of the node.
 func (n *base) Type() reflect.Type {
-return n.nodeType
+if n.nature.Type == nil {
+return anyType
+}
+return n.nature.Type
 }
 
 // SetType sets the type of the node.
 func (n *base) SetType(t reflect.Type) {
-n.nodeType = t
+n.nature.Type = t
 }
 
 // NilNode represents nil.

@@ -235,7 +235,7 @@ func TestBuiltin_errors(t *testing.T) {
 {`bitushr(-5, -2)`, "invalid operation: negative shift count -2 (type int) (1:1)"},
 {`now(nil)`, "invalid number of arguments (expected 0, got 1)"},
 {`date(nil)`, "interface {} is nil, not string (1:1)"},
-{`timezone(nil)`, "interface {} is nil, not string (1:1)"},
+{`timezone(nil)`, "cannot use nil as argument (type string) to call timezone (1:10)"},
 }
 for _, test := range errorTests {
 t.Run(test.input, func(t *testing.T) {