From cbb987111c9b9b2750a853bb41ffc69e9c91c300 Mon Sep 17 00:00:00 2001
From: Oliver Steele <steele@osteele.com>
Date: Thu, 26 Feb 2026 19:22:06 +0800
Subject: [PATCH] feat: add LaxFilters option for undefined filter handling
 (fixes #9)

---
 cmd/liquid/main.go     |  5 +++++
 engine.go              |  7 +++++++
 engine_test.go         | 20 ++++++++++++++++++++
 expressions/config.go  |  3 ++-
 expressions/filters.go |  5 ++++-
 5 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/cmd/liquid/main.go b/cmd/liquid/main.go
index 6c2f7820..3af163fd 100644
--- a/cmd/liquid/main.go
+++ b/cmd/liquid/main.go
@@ -28,6 +28,7 @@ var (
 	env        func() []string = os.Environ
 	bindings   map[string]any  = map[string]any{}
 	strictVars bool
+	laxFilters bool
 )
 
 func main() {
@@ -43,6 +44,7 @@ func main() {
 	var bindEnvs bool
 	cmdLine.BoolVar(&bindEnvs, "env", false, "bind environment variables")
 	cmdLine.BoolVar(&strictVars, "strict", false, "enable strict variable mode in templates")
+	cmdLine.BoolVar(&laxFilters, "lax-filters", false, "ignore undefined filters instead of raising an error")
 
 	err = cmdLine.Parse(os.Args[1:])
 	if err != nil {
@@ -92,6 +94,9 @@ func render() error {
 	if strictVars {
 		e.StrictVariables()
 	}
+	if laxFilters {
+		e.LaxFilters()
+	}
 
 	tpl, err := e.ParseTemplate(buf)
 	if err != nil {
diff --git a/engine.go b/engine.go
index e77591d2..abf1f0e4 100644
--- a/engine.go
+++ b/engine.go
@@ -84,6 +84,13 @@ func (e *Engine) StrictVariables() {
 	e.cfg.StrictVariables = true
 }
 
+// LaxFilters causes the renderer to silently pass through the input value
+// when the template contains an undefined filter, matching Shopify Liquid behavior.
+// By default, undefined filters cause an error.
+func (e *Engine) LaxFilters() {
+	e.cfg.LaxFilters = true
+}
+
 // EnableJekyllExtensions enables Jekyll-specific extensions to Liquid.
 // This includes support for dot notation in assign tags (e.g., {% assign page.canonical_url = value %}).
 // Note: This is not part of the Shopify Liquid standard but is used in Jekyll and Gojekyll.
diff --git a/engine_test.go b/engine_test.go
index 82dc5d20..789a6136 100644
--- a/engine_test.go
+++ b/engine_test.go
@@ -185,6 +185,26 @@ func Test_template_store(t *testing.T) {
 	require.Equal(t, "Message Text: filename from: template.liquid.", out)
 }
 
+func TestEngine_LaxFilters(t *testing.T) {
+	// Default: undefined filters cause an error
+	engine := NewEngine()
+	_, err := engine.ParseAndRenderString(`{{ "hello" | nofilter }}`, emptyBindings)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "undefined filter")
+
+	// LaxFilters: undefined filters pass through the value
+	engine = NewEngine()
+	engine.LaxFilters()
+	out, err := engine.ParseAndRenderString(`{{ "hello" | nofilter }}`, emptyBindings)
+	require.NoError(t, err)
+	require.Equal(t, "hello", out)
+
+	// LaxFilters: defined filters still work
+	out, err = engine.ParseAndRenderString(`{{ "hello" | upcase }}`, emptyBindings)
+	require.NoError(t, err)
+	require.Equal(t, "HELLO", out)
+}
+
 func TestEngine_UnregisterTag(t *testing.T) {
 	engine := NewEngine()
 	engine.RegisterTag("echo", func(c render.Context) (string, error) {
diff --git a/expressions/config.go b/expressions/config.go
index 268a4be9..2378fe78 100644
--- a/expressions/config.go
+++ b/expressions/config.go
@@ -2,7 +2,8 @@ package expressions
 
 // Config holds configuration information for expression interpretation.
 type Config struct {
-	filters map[string]any
+	filters    map[string]any
+	LaxFilters bool
 }
 
 // NewConfig creates a new Config.
diff --git a/expressions/filters.go b/expressions/filters.go
index 636ee5f2..adb79617 100644
--- a/expressions/filters.go
+++ b/expressions/filters.go
@@ -83,7 +83,10 @@ func isClosureInterfaceType(t reflect.Type) bool {
 func (ctx *context) ApplyFilter(name string, receiver valueFn, params []valueFn) (any, error) {
 	filter, ok := ctx.filters[name]
 	if !ok {
-		panic(UndefinedFilter(name))
+		if !ctx.LaxFilters {
+			panic(UndefinedFilter(name))
+		}
+		return receiver(ctx).Interface(), nil
 	}
 
 	fr := reflect.ValueOf(filter)