Allow binds to be provided by a function.

This is useful for situations where the initialisation of some object
should be deferred, eg. when there are distinct "setup" and "use" phases
to a tools lifecycle.

.circleci/config.yml

@@ -12,7 +12,7 @@ jobs:
           command: |
             go get -v github.com/jstemmer/go-junit-report
             go get -v -t -d ./...
-            curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.23.1
+            curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.23.7
             mkdir ~/report
           when: always
       - run:

.golangci.yml

@@ -19,8 +19,6 @@ linters:
 linters-settings:
   govet:
     check-shadowing: true
-  gocyclo:
-    min-complexity: 10
   dupl:
     threshold: 100
   goconst:
@@ -41,3 +39,4 @@ issues:
     - 'composite literal uses unkeyed fields'
     - 'bad syntax for struct tag key'
     - 'bad syntax for struct tag pair'
+    - 'result .* \(error\) is always nil'

callbacks.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 )
 
-type bindings map[reflect.Type]reflect.Value
+type bindings map[reflect.Type]func() (reflect.Value, error)
 
 func (b bindings) String() string {
 	out := []string{}
@@ -18,7 +18,8 @@ func (b bindings) String() string {
 
 func (b bindings) add(values ...interface{}) bindings {
 	for _, v := range values {
-		b[reflect.TypeOf(v)] = reflect.ValueOf(v)
+		v := v
+		b[reflect.TypeOf(v)] = func() (reflect.Value, error) { return reflect.ValueOf(v), nil }
 	}
 	return b
 }
@@ -57,8 +58,12 @@ func callMethod(name string, v, f reflect.Value, bindings bindings) error {
 	}
 	for i := 0; i < t.NumIn(); i++ {
 		pt := t.In(i)
-		if arg, ok := bindings[pt]; ok {
+		if argf, ok := bindings[pt]; ok {
-			in = append(in, arg)
+			argv, err := argf()
+			if err != nil {
+				return err
+			}
+			in = append(in, argv)
 		} else {
 			return fmt.Errorf("couldn't find binding of type %s for parameter %d of %s.%s(), use kong.Bind(%s)", pt, i, v.Type(), name, pt)
 		}

context.go

@@ -91,7 +91,8 @@ func (c *Context) Bind(args ...interface{}) {
 //
 //    BindTo(impl, (*MyInterface)(nil))
 func (c *Context) BindTo(impl, iface interface{}) {
-	c.bindings[reflect.TypeOf(iface).Elem()] = reflect.ValueOf(impl)
+	valueOf := reflect.ValueOf(impl)
+	c.bindings[reflect.TypeOf(iface).Elem()] = func() (reflect.Value, error) { return valueOf, nil }
 }
 
 // Value returns the value for a particular path element.

options.go

@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 	"reflect"
 	"strings"
+
+	"github.com/pkg/errors"
 )
 
 // An Option applies optional changes to the Kong application.
@@ -145,7 +147,33 @@ func Bind(args ...interface{}) Option {
 // 		BindTo(impl, (*iface)(nil))
 func BindTo(impl, iface interface{}) Option {
 	return OptionFunc(func(k *Kong) error {
-		k.bindings[reflect.TypeOf(iface).Elem()] = reflect.ValueOf(impl)
+		valueOf := reflect.ValueOf(impl)
+		k.bindings[reflect.TypeOf(iface).Elem()] = func() (reflect.Value, error) { return valueOf, nil }
+		return nil
+	})
+}
+
+// BindToProvider allows binding of provider functions.
+//
+// This is useful when the Run() function of different commands require different values that may
+// not all be initialisable from the main() function.
+func BindToProvider(provider interface{}) Option {
+	return OptionFunc(func(k *Kong) 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 errors.Errorf("%T must be a function with the signature func()(T, error)", provider)
+		}
+		rt := pv.Type().Out(0)
+		k.bindings[rt] = func() (reflect.Value, error) {
+			out := pv.Call(nil)
+			errv := out[1]
+			var err error
+			if !errv.IsNil() {
+				err = errv.Interface().(error)
+			}
+			return out[0], err
+		}
 		return nil
 	})
 }

options_test.go

@@ -41,3 +41,29 @@ func TestBindTo(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "foo", saw)
 }
+
+type bindToProviderCLI struct {
+	Called bool
+	Cmd    bindToProviderCmd `cmd:""`
+}
+
+type boundThing struct {
+}
+
+type bindToProviderCmd struct{}
+
+func (*bindToProviderCmd) Run(cli *bindToProviderCLI, b *boundThing) error {
+	cli.Called = true
+	return nil
+}
+
+func TestBindToProvider(t *testing.T) {
+	var cli bindToProviderCLI
+	app, err := New(&cli, BindToProvider(func() (*boundThing, error) { return &boundThing{}, nil }))
+	require.NoError(t, err)
+	ctx, err := app.Parse([]string{"cmd"})
+	require.NoError(t, err)
+	err = ctx.Run()
+	require.NoError(t, err)
+	require.True(t, cli.Called)
+}