feat: comment template function

README.md

@@ -377,6 +377,7 @@ For example, this is a JSON version of an emitted RuntimeContainer struct:
 - [Functions from Sprig v3](https://masterminds.github.io/sprig/), except for those that have the same name as one of the following functions.
 - _`closest $array $value`_: Returns the longest matching substring in `$array` that matches `$value`
 - _`coalesce ...`_: Returns the first non-nil argument.
+- _`comment $delimiter $string`_: Returns `$string` with each line prefixed by `$delimiter` (helpful for debugging combined with Sprig `toPrettyJson`: `{{ toPrettyJson $ | comment "#" }}`).
 - _`contains $map $key`_: Returns `true` if `$map` contains `$key`. Takes maps from `string` to any type.
 - _`dir $path`_: Returns an array of filenames in the specified `$path`.
 - _`exists $path`_: Returns `true` if `$path` refers to an existing file or directory. Takes a string.

internal/template/functions.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"os"
 	"reflect"
+	"regexp"
 	"strings"
 )
 
@@ -57,6 +58,12 @@ func intersect(l1, l2 []string) []string {
 	return keys
 }
 
+// comment prefix each line of the source string with the provided comment delimiter string
+func comment(delimiter string, source string) string {
+	regexPattern := regexp.MustCompile(`(?m)^`)
+	return regexPattern.ReplaceAllString(source, delimiter)
+}
+
 func contains(input interface{}, key interface{}) bool {
 	if input == nil {
 		return false

internal/template/functions_test.go

@@ -12,6 +12,24 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestComment(t *testing.T) {
+	env := map[string]string{
+		"bar": "baz",
+		"foo": "test",
+	}
+
+	expected := `# {
+#   "bar": "baz",
+#   "foo": "test"
+# }`
+
+	tests := templateTestList{
+		{`{{toPrettyJson . | comment "# "}}`, env, expected},
+	}
+
+	tests.run(t)
+}
+
 func TestContainsString(t *testing.T) {
 	env := map[string]string{
 		"PORT": "1234",

internal/template/template.go

@@ -60,6 +60,7 @@ func newTemplate(name string) *template.Template {
 	tmpl.Funcs(sprig.TxtFuncMap()).Funcs(template.FuncMap{
 		"closest":                 arrayClosest,
 		"coalesce":                coalesce,
+		"comment":                 comment,
 		"contains":                contains,
 		"dir":                     dirList,
 		"eval":                    eval,