From a9b2aa00043711b5bf17217dc48e43025f1b2480 Mon Sep 17 00:00:00 2001
From: Cody Krieger <cody@krieger.io>
Date: Sat, 5 Oct 2024 15:28:07 -0700
Subject: [PATCH] implement support for else clauses in for loops. fixes
 osteele/liquid#46

---
 tags/iteration_tags.go      | 45 ++++++++++++++++++++++++-------------
 tags/iteration_tags_test.go |  3 +++
 tags/standard_tags.go       |  2 +-
 3 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/tags/iteration_tags.go b/tags/iteration_tags.go
index 702e443..80c9ed0 100644
--- a/tags/iteration_tags.go
+++ b/tags/iteration_tags.go
@@ -67,7 +67,34 @@ func loopTagCompiler(node render.BlockNode) (func(io.Writer, render.Context) err
 	if err != nil {
 		return nil, err
 	}
-	return loopRenderer{stmt.Loop, node.Name}.render, nil
+
+	return func(w io.Writer, ctx render.Context) error {
+		// loop modifiers
+		val, err := ctx.Evaluate(stmt.Loop.Expr)
+		if err != nil {
+			return err
+		}
+
+		iter := makeIterator(val)
+		if iter == nil {
+			return nil
+		}
+
+		iter, err = applyLoopModifiers(stmt.Loop, ctx, iter)
+		if err != nil {
+			return err
+		}
+
+		if len(node.Clauses) > 1 {
+			return fmt.Errorf("for loops accept at most one else clause")
+		}
+
+		if iter.Len() == 0 && len(node.Clauses) == 1 && node.Clauses[0].Name == "else" {
+			return ctx.RenderBlock(w, node.Clauses[0])
+		}
+
+		return loopRenderer{stmt.Loop, node.Name}.render(iter, w, ctx)
+	}, nil
 }
 
 type loopRenderer struct {
@@ -75,21 +102,7 @@ type loopRenderer struct {
 	tagName string
 }
 
-func (loop loopRenderer) render(w io.Writer, ctx render.Context) error {
-	// loop modifiers
-	val, err := ctx.Evaluate(loop.Expr)
-	if err != nil {
-		return err
-	}
-	iter := makeIterator(val)
-	if iter == nil {
-		return nil
-	}
-	iter, err = applyLoopModifiers(loop.Loop, ctx, iter)
-	if err != nil {
-		return err
-	}
-
+func (loop loopRenderer) render(iter iterable, w io.Writer, ctx render.Context) error {
 	// loop decorator
 	decorator, err := makeLoopDecorator(loop, ctx)
 	if err != nil {
diff --git a/tags/iteration_tags_test.go b/tags/iteration_tags_test.go
index b91dc6c..7b81a87 100644
--- a/tags/iteration_tags_test.go
+++ b/tags/iteration_tags_test.go
@@ -17,6 +17,7 @@ import (
 
 var iterationTests = []struct{ in, expected string }{
 	{`{% for a in array %}{{ a }} {% endfor %}`, "first second third "},
+	{`{% for a in array %}{{ a }} {% else %}else{% endfor %}`, "first second third "},
 	{`{% for a in nil %}{{ a }}.{% endfor %}`, ""},
 	{`{% for a in false %}{{ a }}.{% endfor %}`, ""},
 	{`{% for a in 2 %}{{ a }}.{% endfor %}`, ""},
@@ -37,6 +38,7 @@ var iterationTests = []struct{ in, expected string }{
 	{`{% for a in array offset: loopmods["offset"] %}{{ a }}.{% endfor %}`, "second.third."},
 	{`{% for a in array reversed limit: 1 %}{{ a }}.{% endfor %}`, "third."},
 	{`{% for a in array limit: 0 %}{{ a }}.{% endfor %}`, ""},
+	{`{% for a in array limit: 0 %}{{ a }}.{% else %}ELSE{% endfor %}`, "ELSE"},
 	{`{% for a in array offset: 3 %}{{ a }}.{% endfor %}`, ""},
 	{`{% for a in array offset: 10 %}{{ a }}.{% endfor %}`, ""},
 	// TODO investigate how these combine; does it depend on the order?
@@ -126,6 +128,7 @@ var iterationErrorTests = []struct{ in, expected string }{
 	{`{% cycle 'a', 'b' %}`, "cycle must be within a forloop"},
 	{`{% for a in array | undefined_filter %}{% endfor %}`, "undefined filter"},
 	{`{% for a in array %}{{ a | undefined_filter }}{% endfor %}`, "undefined filter"},
+	{`{% for a in array %}{% else %}{% else %}{% endfor %}`, "for loops accept at most one else clause"},
 }
 
 var iterationTestBindings = map[string]interface{}{
diff --git a/tags/standard_tags.go b/tags/standard_tags.go
index 76fcf78..b45b654 100644
--- a/tags/standard_tags.go
+++ b/tags/standard_tags.go
@@ -22,7 +22,7 @@ func AddStandardTags(c render.Config) {
 	c.AddBlock("capture").Compiler(captureTagCompiler)
 	c.AddBlock("case").Clause("when").Clause("else").Compiler(caseTagCompiler)
 	c.AddBlock("comment")
-	c.AddBlock("for").Compiler(loopTagCompiler)
+	c.AddBlock("for").Clause("else").Compiler(loopTagCompiler)
 	c.AddBlock("if").Clause("else").Clause("elsif").Compiler(ifTagCompiler(true))
 	c.AddBlock("raw")
 	c.AddBlock("tablerow").Compiler(loopTagCompiler)