feat: add include template function

docs/site/docs/usage.mdx

@@ -89,7 +89,7 @@ The rest of the files are rendered from the templates. You can edit the template
 
 ## Templating
 
-Templates are done by using [Go templates](https://pkg.go.dev/text/template). Examples of how to write these templates can be found in [this article](https://www.digitalocean.com/community/tutorials/how-to-use-templates-in-go#step-4-writing-a-template) and in the [example recipe](https://github.com/futurice/jalapeno/blob/main/examples/variable-types/templates/README.md?plain=1).
+Templates are done by using [Go templates](https://pkg.go.dev/text/template) extended with [sprig functions](https://masterminds.github.io/sprig/). Examples of how to write these templates can be found in [this article](https://www.digitalocean.com/community/tutorials/how-to-use-templates-in-go#step-4-writing-a-template) and in the [example recipe](https://github.com/futurice/jalapeno/blob/main/examples/variable-types/templates/README.md?plain=1).
 
 The following context is available on the templates:
 

pkg/engine/engine.go

@@ -16,7 +16,7 @@ func New() Engine {
 
 func (e Engine) Render(templates map[string][]byte, values map[string]interface{}) (map[string][]byte, error) {
 	t := template.New("gotpl")
-	t.Funcs(funcMap())
+	t.Funcs(funcMap(t))
 
 	rendered := make(map[string][]byte)
 

pkg/engine/engine_test.go

@@ -30,13 +30,14 @@ func TestRender(t *testing.T) {
 		{
 			"macros",
 			map[string][]byte{
+				"templates/main":    []byte("{{ template \"helper1\" }} {{ template \"helper2\" }} {{ include \"helper3\" . | upper }}"),
 				"templates/helper1": []byte("{{ define \"helper1\" }}ONE{{ end }}"),
-				"templates/main":    []byte("{{ template \"helper1\" }} {{ template \"helper2\" }}"),
 				"templates/helper2": []byte("{{ define \"helper2\" }}TWO{{ end }}"),
+				"templates/helper3": []byte("{{ define \"helper3\" }}three{{ end }}"),
 			},
 			map[string]interface{}{},
 			map[string][]byte{
-				"templates/main": []byte("ONE TWO"),
+				"templates/main": []byte("ONE TWO THREE"),
 			},
 		},
 	}

pkg/engine/funcs.go

@@ -1,6 +1,7 @@
 package engine
 
 import (
+	"fmt"
 	"math/big"
 	"math/rand"
 	"strings"
@@ -11,10 +12,12 @@ import (
 
 var prngs = make(map[string]*rand.Rand)
 
-func funcMap() template.FuncMap {
+func funcMap(t *template.Template) template.FuncMap {
 	f := sprig.TxtFuncMap()
+	includedNames := make(map[string]int)
 
-	// Add additional template functions here
+	// Custom template functions
+	f["include"] = includeFun(t, includedNames)
 	f["stableRandomAlphanumeric"] = stableRandomAlphanumeric
 
 	return f
@@ -23,6 +26,7 @@ func funcMap() template.FuncMap {
 const lowerAlpha = "abcdefghijlkmnopqrstuvwxyz"
 const number = "0123456789"
 const alphanumeric = lowerAlpha + number
+const recursionMaxNums = 100
 
 // Generate a pseudo-random alphanumeric string of the given length
 // such that the sequence of strings generated by successive calls
@@ -56,3 +60,20 @@ func prngForStr(str string) *rand.Rand {
 func resetRngs() {
 	prngs = make(map[string]*rand.Rand)
 }
+
+func includeFun(t *template.Template, includedNames map[string]int) func(string, interface{}) (string, error) {
+	return func(name string, data interface{}) (string, error) {
+		var buf strings.Builder
+		if v, ok := includedNames[name]; ok {
+			if v > recursionMaxNums {
+				return "", fmt.Errorf("unable to execute template: maximum recursion depth reached for template block \"%s\"", name)
+			}
+			includedNames[name]++
+		} else {
+			includedNames[name] = 1
+		}
+		err := t.ExecuteTemplate(&buf, name, data)
+		includedNames[name]--
+		return buf.String(), err
+	}
+}