From 51cffd25efb64d6f424f65773e387ca45ceb26a1 Mon Sep 17 00:00:00 2001
From: Tony Holdstock-Brown <tonyhb@gmail.com>
Date: Wed, 23 Oct 2024 12:50:02 -0700
Subject: [PATCH] Handle priting of nils in a JSON compatible manner

This removes the <nil> formatting from .String()
---
 expr_test.go   | 14 ++++++++++++++
 parser.go      |  7 +++++++
 parser_test.go |  4 ++--
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/expr_test.go b/expr_test.go
index 1c31c73..9e185c2 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -754,6 +754,20 @@ func TestAddRemove(t *testing.T) {
 		require.EqualValues(t, 1, count)
 		require.EqualValues(t, 1, len(eval))
 		require.NoError(t, err)
+
+		// Matching this expr should now fail.
+		eval, count, err = e.Evaluate(ctx, map[string]any{
+			"event": map[string]any{
+				"data": map[string]any{
+					"foo": "yea",
+					"bar": "baz",
+				},
+			},
+		})
+
+		require.EqualValues(t, 1, count)
+		require.EqualValues(t, 0, len(eval))
+		require.NoError(t, err)
 	})
 }
 
diff --git a/parser.go b/parser.go
index 777e5bb..f8ef615 100644
--- a/parser.go
+++ b/parser.go
@@ -326,6 +326,13 @@ func (p Predicate) String() string {
 	switch str := p.Literal.(type) {
 	case string:
 		return fmt.Sprintf("%s %s %v", p.Ident, strings.ReplaceAll(p.Operator, "_", ""), strconv.Quote(str))
+	case nil:
+		if p.LiteralIdent == nil {
+			// print `foo == null` instead of `foo == <nil>`, the Golang default.
+			// We onyl do this if we're not comparing to an identifier.
+			return fmt.Sprintf("%s %s null", p.Ident, strings.ReplaceAll(p.Operator, "_", ""))
+		}
+		return fmt.Sprintf("%s %s %v", p.Ident, strings.ReplaceAll(p.Operator, "_", ""), lit)
 	default:
 		return fmt.Sprintf("%s %s %v", p.Ident, strings.ReplaceAll(p.Operator, "_", ""), lit)
 	}
diff --git a/parser_test.go b/parser_test.go
index 52b327c..77534aa 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -1060,7 +1060,7 @@ func TestParse(t *testing.T) {
 		tests := []parseTestInput{
 			{
 				input:  `has(event.name)`,
-				output: "name select <nil>",
+				output: "name select null",
 				expected: ParsedExpression{
 					Root: Node{
 						GroupID: newGroupID(1),
@@ -1079,7 +1079,7 @@ func TestParse(t *testing.T) {
 			},
 			{
 				input:  `event.data.num.filter(x, x >= 10)`,
-				output: "x comprehension <nil>",
+				output: "x comprehension null",
 				expected: ParsedExpression{
 					Root: Node{
 						GroupID: newGroupID(1),