feat: support recursive injection of provider parameters

This allows provider functions to accept parameters that are injected by other
bindings or binding providers, eg. call the provider function with the root CLI
struct (which is automatically bound by Kong):

  kong.BindToProvider(func(cli *CLI) (*Injected, error) { ... })

callbacks.go

@@ -6,7 +6,10 @@ import (
 	"strings"
 )
 
-type bindings map[reflect.Type]func() (reflect.Value, error)
+// A map of type to function that returns a value of that type.
+//
+// The function should have the signature func(...) (T, error). Arguments are recursively resolved.
+type bindings map[reflect.Type]any
 
 func (b bindings) String() string {
 	out := []string{}
@@ -19,32 +22,23 @@ func (b bindings) String() string {
 func (b bindings) add(values ...interface{}) bindings {
 	for _, v := range values {
 		v := v
-		b[reflect.TypeOf(v)] = func() (reflect.Value, error) { return reflect.ValueOf(v), nil }
+		b[reflect.TypeOf(v)] = func() (any, error) { return v, nil }
 	}
 	return b
 }
 
 func (b bindings) addTo(impl, iface interface{}) {
-	valueOf := reflect.ValueOf(impl)
-	b[reflect.TypeOf(iface).Elem()] = func() (reflect.Value, error) { return valueOf, nil }
+	b[reflect.TypeOf(iface).Elem()] = func() (any, error) { return impl, nil }
 }
 
 func (b bindings) addProvider(provider interface{}) error {
 	pv := reflect.ValueOf(provider)
 	t := pv.Type()
-	if t.Kind() != reflect.Func || t.NumIn() != 0 || 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 || 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)
 	}
 	rt := pv.Type().Out(0)
-	b[rt] = func() (reflect.Value, error) {
-		out := pv.Call(nil)
-		errv := out[1]
-		var err error
-		if !errv.IsNil() {
-			err = errv.Interface().(error) //nolint
-		}
-		return out[0], err
-	}
+	b[rt] = provider
 	return nil
 }
 
@@ -101,15 +95,19 @@ func callAnyFunction(f reflect.Value, bindings bindings) (out []any, err error)
 	t := f.Type()
 	for i := 0; i < t.NumIn(); i++ {
 		pt := t.In(i)
-		if argf, ok := bindings[pt]; ok {
-			argv, err := argf()
-			if err != nil {
-				return nil, err
-			}
-			in = append(in, argv)
-		} else {
+		argf, ok := bindings[pt]
+		if !ok {
 			return nil, fmt.Errorf("couldn't find binding of type %s for parameter %d of %s(), use kong.Bind(%s)", pt, i, t, pt)
 		}
+		// Recursively resolve binding functions.
+		argv, err := callAnyFunction(reflect.ValueOf(argf), bindings)
+		if err != nil {
+			return nil, fmt.Errorf("%s: %w", pt, err)
+		}
+		if ferrv := reflect.ValueOf(argv[len(argv)-1]); ferrv.IsValid() && !ferrv.IsNil() {
+			return nil, ferrv.Interface().(error) //nolint:forcetypeassert
+		}
+		in = append(in, reflect.ValueOf(argv[0]))
 	}
 	outv := f.Call(in)
 	out = make([]any, len(outv))

options.go

@@ -208,7 +208,11 @@ func BindTo(impl, iface interface{}) Option {
 	})
 }
 
-// BindToProvider allows binding of provider functions.
+// BindToProvider binds an injected value to a provider function.
+//
+// The provider function must have the signature:
+//
+//	func() (interface{}, error)
 //
 // This is useful when the Run() function of different commands require different values that may
 // not all be initialisable from the main() function.

options_test.go

@@ -89,11 +89,13 @@ func TestCallbackCustomError(t *testing.T) {
 }
 
 type bindToProviderCLI struct {
+	Filled bool `default:"true"`
 	Called bool
 	Cmd    bindToProviderCmd `cmd:""`
 }
 
 type boundThing struct {
+	Filled bool
 }
 
 type bindToProviderCmd struct{}
@@ -105,7 +107,10 @@ func (*bindToProviderCmd) Run(cli *bindToProviderCLI, b *boundThing) error {
 
 func TestBindToProvider(t *testing.T) {
 	var cli bindToProviderCLI
-	app, err := New(&cli, BindToProvider(func() (*boundThing, error) { return &boundThing{}, nil }))
+	app, err := New(&cli, BindToProvider(func(cli *bindToProviderCLI) (*boundThing, error) {
+		assert.True(t, cli.Filled, "CLI struct should have already been populated by Kong")
+		return &boundThing{Filled: cli.Filled}, nil
+	}))
 	assert.NoError(t, err)
 	ctx, err := app.Parse([]string{"cmd"})
 	assert.NoError(t, err)