feat: support go template headers

template.go

@@ -1,6 +1,7 @@
 package gomplate
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -203,6 +204,11 @@ func goTemplate(template Template, environment map[string]any) (string, error) {
 	}
 
 	if tpl == nil {
+		template, err := parseAndStripTemplateHeader(template)
+		if err != nil {
+			return "", err
+		}
+
 		tpl = gotemplate.New("")
 		if template.LeftDelim != "" {
 			tpl = tpl.Delims(template.LeftDelim, template.RightDelim)
@@ -215,7 +221,7 @@ func goTemplate(template Template, environment map[string]any) (string, error) {
 		for k, v := range template.Functions {
 			funcs[k] = v
 		}
-		var err error
+
 		tpl, err = tpl.Funcs(funcs).Parse(template.Template)
 		if err != nil {
 			return "", err
@@ -249,3 +255,50 @@ func LoadSharedLibrary(source string) error {
 	registry.Register(func() string { return string(data) })
 	return nil
 }
+
+func parseAndStripTemplateHeader(template Template) (Template, error) {
+	fm, content := extractFrontmatterAndContent(template.Template)
+	if fm == "" {
+		return template, nil
+	}
+
+	template.Template = content
+	scanner := bufio.NewScanner(strings.NewReader(fm))
+	for scanner.Scan() {
+		line := scanner.Text()
+		split := strings.SplitN(line, "=", 2)
+		if len(split) != 2 {
+			return template, fmt.Errorf("invalid header: %s", line)
+		}
+
+		switch split[0] {
+		case "right-delim":
+			template.RightDelim = split[1]
+		case "left-delim":
+			template.LeftDelim = split[1]
+		}
+	}
+
+	return template, nil
+}
+
+func extractFrontmatterAndContent(input string) (string, string) {
+	const delimiter = "---"
+
+	input = strings.TrimSpace(input)
+
+	if !strings.HasPrefix(input, delimiter) {
+		return "", input
+	}
+
+	endIndex := strings.Index(input[len(delimiter):], delimiter)
+	if endIndex == -1 {
+		return "", input
+	}
+
+	frontmatterEndIndex := endIndex + 2*len(delimiter)
+	frontmatter := strings.TrimSpace(input[len(delimiter) : endIndex+len(delimiter)])
+	content := strings.TrimSpace(input[frontmatterEndIndex:])
+
+	return frontmatter, content
+}

tests/gomplate_test.go

@@ -1,6 +1,7 @@
 package tests
 
 import (
+	"os"
 	"testing"
 	"time"
 
@@ -80,3 +81,30 @@ func TestGomplate(t *testing.T) {
 		})
 	}
 }
+
+func TestGomplateHeaders(t *testing.T) {
+	tests := []struct {
+		env      map[string]interface{}
+		template string
+		out      string
+	}{
+		{map[string]interface{}{"name": "world"}, readFile(t, "testdata/template-header.txt"), "Hello, world"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.template, func(t *testing.T) {
+			out, err := gomplate.RunTemplate(tc.env, gomplate.Template{
+				Template: tc.template,
+			})
+			assert.ErrorIs(t, err, nil)
+			assert.Equal(t, tc.out, out)
+		})
+	}
+}
+
+func readFile(t *testing.T, filename string) string {
+	t.Helper()
+	data, err := os.ReadFile(filename)
+	assert.ErrorIs(t, err, nil)
+	return string(data)
+}

tests/testdata/template-header.txt

@@ -0,0 +1,5 @@
+---
+left-delim=$[[
+right-delim=]]
+---
+Hello, $[[.name]]