Add better errors for nil dereference

Author: antonmedv

eval.go

@@ -238,6 +238,9 @@ func (n propertyNode) Eval(env interface{}) (interface{}, error) {
 	}
 	p, ok := extract(v, n.property)
 	if !ok {
+		if isNil(v) {
+			return nil, fmt.Errorf("%v is nil", n.node)
+		}
 		return nil, fmt.Errorf("%v undefined (type %T has no field %v)", n, v, n.property)
 	}
 	return p, nil

eval_test.go

@@ -447,6 +447,21 @@ var evalErrorTests = []evalErrorTest{
 		nil,
 		"too many arguments: len(a, b)",
 	},
+	{
+		"Foo.Map",
+		struct{ Foo map[string]int }{Foo: nil},
+		"Foo is nil",
+	},
+	{
+		"Foo.Bar",
+		struct{ Foo *struct{ Bar bool } }{Foo: nil},
+		"Foo is nil",
+	},
+	{
+		"Foo.Panic",
+		struct{ Foo interface{} }{Foo: nil},
+		"Foo is nil",
+	},
 }
 
 func TestEval(t *testing.T) {

runtime.go

@@ -120,3 +120,8 @@ func contains(needle interface{}, array interface{}) (bool, error) {
 	}
 	return false, nil
 }
+
+func isNil(val interface{}) bool {
+	v := reflect.ValueOf(val)
+	return !v.IsValid() || v.IsNil()
+}