From e21d2a736da6b81c76d80d023d11cf04b57354d6 Mon Sep 17 00:00:00 2001
From: Oliver Steele <steele@osteele.com>
Date: Tue, 27 Jun 2017 07:43:42 -0400
Subject: [PATCH] Add public DefineTag

---
 chunks/definitions.go  | 17 +++++++++++------
 liquid.go => engine.go | 35 ++++++++---------------------------
 template.go            | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 33 deletions(-)
 rename liquid.go => engine.go (67%)
 create mode 100644 template.go

diff --git a/chunks/definitions.go b/chunks/definitions.go
index b148509..b9295e4 100644
--- a/chunks/definitions.go
+++ b/chunks/definitions.go
@@ -18,10 +18,15 @@ func assignTagDef(source string) (func(io.Writer, Context) error, error) {
 	}, nil
 }
 
-func FindTagDefinition(name string) (TagDefinition, bool) {
-	switch name {
-	case "assign":
-		return assignTagDef, true
-	}
-	return nil, false
+var tagDefinitions = map[string]TagDefinition{
+	"assign": assignTagDef,
+}
+
+func DefineTag(name string, td TagDefinition) {
+	tagDefinitions[name] = td
+}
+
+func FindTagDefinition(name string) (TagDefinition, bool) {
+	td, ok := tagDefinitions[name]
+	return td, ok
 }
diff --git a/liquid.go b/engine.go
similarity index 67%
rename from liquid.go
rename to engine.go
index 3780513..932e8b2 100644
--- a/liquid.go
+++ b/engine.go
@@ -6,7 +6,7 @@ It's intended for use in for use in https://github.com/osteele/gojekyll.
 package liquid
 
 import (
-	"bytes"
+	"io"
 
 	"github.com/osteele/liquid/chunks"
 )
@@ -15,18 +15,14 @@ import (
 //
 // In the future, it will be configured with additional tags, filters, and the {%include%} search path.
 type Engine interface {
+	DefineTag(string, func(form string) (func(io.Writer, chunks.Context) error, error))
+
 	ParseTemplate(text []byte) (Template, error)
 	ParseAndRender(text []byte, scope map[string]interface{}) ([]byte, error)
 	ParseAndRenderString(text string, scope map[string]interface{}) (string, error)
 }
 
-// Template renders a template according to scope.
-//
-// Scope is a map of liquid variable names to objects.
-type Template interface {
-	Render(scope map[string]interface{}) ([]byte, error)
-	RenderString(scope map[string]interface{}) (string, error)
-}
+type TagDefinition func(expr string) (func(io.Writer, chunks.Context) error, error)
 
 type engine struct{}
 
@@ -39,6 +35,10 @@ func NewEngine() Engine {
 	return engine{}
 }
 
+func (e engine) DefineTag(name string, td func(form string) (func(io.Writer, chunks.Context) error, error)) {
+	chunks.DefineTag(name, chunks.TagDefinition(td))
+}
+
 func (e engine) ParseTemplate(text []byte) (Template, error) {
 	tokens := chunks.Scan(string(text), "")
 	ast, err := chunks.Parse(tokens)
@@ -66,22 +66,3 @@ func (e engine) ParseAndRenderString(text string, scope map[string]interface{})
 	}
 	return string(b), nil
 }
-
-// Render applies the template to the scope.
-func (t *template) Render(scope map[string]interface{}) ([]byte, error) {
-	buf := new(bytes.Buffer)
-	err := t.ast.Render(buf, chunks.NewContext(scope))
-	if err != nil {
-		return nil, err
-	}
-	return buf.Bytes(), nil
-}
-
-// RenderString is a convenience wrapper for Render, that has string input and output.
-func (t *template) RenderString(scope map[string]interface{}) (string, error) {
-	b, err := t.Render(scope)
-	if err != nil {
-		return "", err
-	}
-	return string(b), nil
-}
diff --git a/template.go b/template.go
new file mode 100644
index 0000000..9150a38
--- /dev/null
+++ b/template.go
@@ -0,0 +1,34 @@
+package liquid
+
+import (
+	"bytes"
+
+	"github.com/osteele/liquid/chunks"
+)
+
+// Template renders a template according to scope.
+//
+// Scope is a map of liquid variable names to objects.
+type Template interface {
+	Render(scope map[string]interface{}) ([]byte, error)
+	RenderString(scope map[string]interface{}) (string, error)
+}
+
+// Render applies the template to the scope.
+func (t *template) Render(scope map[string]interface{}) ([]byte, error) {
+	buf := new(bytes.Buffer)
+	err := t.ast.Render(buf, chunks.NewContext(scope))
+	if err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// RenderString is a convenience wrapper for Render, that has string input and output.
+func (t *template) RenderString(scope map[string]interface{}) (string, error) {
+	b, err := t.Render(scope)
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}