feat(dotprompt-go): add RegisterHelper method for custom template helpers

go/dotprompt/dotprompt.go

@@ -159,7 +159,19 @@ func (dp *Dotprompt) DefinePartial(name string, source string, tpl *raymond.Temp
 	return nil
 }
 
-// TODO(#501): Add register helpers
+// RegisterHelper registers a custom helper function for use in templates,
+// mirroring the JS implementation's defineHelper API. The helper is applied
+// the next time a template is compiled via Compile or Render.
+func (dp *Dotprompt) RegisterHelper(name string, helper any) *Dotprompt {
+	if dp.Helpers == nil {
+		dp.Helpers = make(map[string]any)
+	}
+	dp.Helpers[name] = helper
+	return dp
+}
+
+// RegisterHelpers applies all pending custom helpers (from RegisterHelper or
+// DotpromptOptions.Helpers) plus the built-in template helpers to tpl.
 func (dp *Dotprompt) RegisterHelpers(tpl *raymond.Template) error {
 	if dp.Helpers != nil {
 		for key, helper := range dp.Helpers {

go/dotprompt/dotprompt_test.go

@@ -112,6 +112,42 @@ func TestDefinePartial(t *testing.T) {
 	}
 }
 
+// TestRegisterHelper tests the public RegisterHelper API for registering a
+// custom helper before compiling a template.
+func TestRegisterHelper(t *testing.T) {
+	dp := NewDotprompt(nil)
+
+	shoutHelperFunc := func(s string) string {
+		return strings.ToUpper(s) + "!"
+	}
+
+	returned := dp.RegisterHelper("shout", shoutHelperFunc)
+	if returned != dp {
+		t.Fatalf("Expected RegisterHelper to return the same *Dotprompt for chaining")
+	}
+
+	if _, ok := dp.Helpers["shout"]; !ok {
+		t.Fatalf("Expected helper 'shout' to be present in dp.Helpers")
+	}
+
+	tpl, err := raymond.Parse("{{shout 'hello'}}")
+	if err != nil {
+		t.Fatalf("Failed to parse template: %v", err)
+	}
+
+	if err = dp.RegisterHelpers(tpl); err != nil {
+		t.Fatalf("RegisterHelpers failed: %v", err)
+	}
+
+	result, err := tpl.Exec(nil)
+	if err != nil {
+		t.Fatalf("Template execution failed: %v", err)
+	}
+	if result != "HELLO!" {
+		t.Errorf("Expected template output 'HELLO!', got '%s'", result)
+	}
+}
+
 // TestRegisterHelpers tests registering helpers from options and built-ins.
 func TestRegisterHelpers(t *testing.T) {
 	optionHelperName := "optionHelper"