feat: allow use of providers that don't return errors

Author: alecthomas

callbacks.go

@@ -34,8 +34,17 @@ func (b bindings) addTo(impl, iface any) {
 func (b bindings) addProvider(provider any) error {
 	pv := reflect.ValueOf(provider)
 	t := pv.Type()
-	if t.Kind() != reflect.Func || t.NumOut() != 2 || t.Out(1) != reflect.TypeOf((*error)(nil)).Elem() {
-		return fmt.Errorf("%T must be a function with the signature func(...)(T, error)", provider)
+	if t.Kind() != reflect.Func {
+		return fmt.Errorf("%T must be a function", provider)
+	}
+
+	if t.NumOut() == 0 {
+		return fmt.Errorf("%T must be a function with the signature func(...)(T, error) or func(...) T", provider)
+	}
+	if t.NumOut() == 2 {
+		if t.Out(1) != reflect.TypeOf((*error)(nil)).Elem() {
+			return fmt.Errorf("missing error; %T must be a function with the signature func(...)(T, error) or func(...) T", provider)
+		}
 	}
 	rt := pv.Type().Out(0)
 	b[rt] = provider
@@ -143,7 +152,7 @@ func callAnyFunction(f reflect.Value, bindings bindings) (out []any, err error)
 		if err != nil {
 			return nil, fmt.Errorf("%s: %w", pt, err)
 		}
-		if ferrv := reflect.ValueOf(argv[len(argv)-1]); ferrv.IsValid() && !ferrv.IsNil() {
+		if ferrv := reflect.ValueOf(argv[len(argv)-1]); ferrv.IsValid() && ferrv.Type().Implements(callbackReturnSignature) && !ferrv.IsNil() {
 			return nil, ferrv.Interface().(error) //nolint:forcetypeassert
 		}
 		in = append(in, reflect.ValueOf(argv[0]))

context.go

@@ -119,6 +119,9 @@ func (c *Context) BindTo(impl, iface any) {
 //
 // This is useful when the Run() function of different commands require different values that may
 // not all be initialisable from the main() function.
+//
+// "provider" must be a function with the signature func(...) (T, error) or func(...) T, where
+// ... will be recursively injected with bound values.
 func (c *Context) BindToProvider(provider any) error {
 	return c.bindings.addProvider(provider)
 }

kong_test.go

@@ -2521,3 +2521,23 @@ func TestIssue483EmptyRootNodeNoRun(t *testing.T) {
 	assert.Error(t, err)
 	assert.Contains(t, err.Error(), "no command selected")
 }
+
+type providerWithoutErrorCLI struct {
+}
+
+func (p *providerWithoutErrorCLI) Run(name string) error {
+	if name == "Bob" {
+		return nil
+	}
+	return fmt.Errorf("name %s is not Bob", name)
+}
+
+func TestProviderWithoutError(t *testing.T) {
+	k := mustNew(t, &providerWithoutErrorCLI{})
+	kctx, err := k.Parse(nil)
+	assert.NoError(t, err)
+	err = kctx.BindToProvider(func() string { return "Bob" })
+	assert.NoError(t, err)
+	err = kctx.Run()
+	assert.NoError(t, err)
+}