Add a default-enabled EnvResolver for the env tag.

kong.go

@@ -52,12 +52,13 @@ type Kong struct {
 // See the README (https://github.com/alecthomas/kong) for usage instructions.
 func New(grammar interface{}, options ...Option) (*Kong, error) {
 	k := &Kong{
-		Exit:     os.Exit,
+		Exit:      os.Exit,
-		Stdout:   os.Stdout,
+		Stdout:    os.Stdout,
-		Stderr:   os.Stderr,
+		Stderr:    os.Stderr,
-		before:   map[reflect.Value]HookFunc{},
+		before:    map[reflect.Value]HookFunc{},
-		registry: NewRegistry().RegisterDefaults(),
+		registry:  NewRegistry().RegisterDefaults(),
-		help:     PrintHelp,
+		help:      PrintHelp,
+		resolvers: []ResolverFunc{EnvResolver()},
 	}
 
 	for _, option := range options {

options.go

@@ -97,6 +97,13 @@ func Help(help HelpFunction) Option {
 	}
 }
 
+// ClearResolvers clears all existing resolvers.
+func ClearResolvers() Option {
+	return func(k *Kong) {
+		k.resolvers = nil
+	}
+}
+
 // Resolver registers flag resolvers.
 func Resolver(resolvers ...ResolverFunc) Option {
 	return func(k *Kong) {

resolver.go

@@ -62,8 +62,9 @@ func jsonDecodeValue(sep rune, value interface{}) (string, error) {
 }
 
 // PerFlagEnvResolver automatically determines environment variables based on the name of each flag, transformed to
-// uppercase and underscored, e.g. `my-flag` -> `MY_FLAG` The environment variable key can be overridden with the `env`
+// uppercase and underscored, e.g. `my-flag` -> `MY_FLAG`.
-// tag.
+//
+// The environment variable key can be overridden with the `env:"<name>"` tag.
 func PerFlagEnvResolver(prefix string) ResolverFunc {
 	return func(context *Context, parent *Path, flag *Flag) (string, error) {
 		v, _ := os.LookupEnv(envString(prefix, flag))
@@ -71,6 +72,19 @@ func PerFlagEnvResolver(prefix string) ResolverFunc {
 	}
 }
 
+// EnvResolver resolves flag values using the `env:"<name>"` tag. It ignores flags without this tag.
+//
+// This resolver is installed by default.
+func EnvResolver() ResolverFunc {
+	return func(context *Context, parent *Path, flag *Flag) (string, error) {
+		if flag.Tag.Env == "" {
+			return "", nil
+		}
+		v, _ := os.LookupEnv(flag.Tag.Env)
+		return v, nil
+	}
+}
+
 func envString(prefix string, flag *Flag) string {
 	if env, ok := flag.Tag.Get("env"); ok {
 		return env

resolver_test.go

@@ -235,3 +235,14 @@ func TestResolverSatisfiesRequired(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 1, cli.Int)
 }
+
+func TestEnvResolver(t *testing.T) {
+	var cli struct {
+		Int int `env:"SOME_ENVAR"`
+	}
+	restoreEnv := tempEnv(envMap{"SOME_ENVAR": "12"})
+	defer restoreEnv()
+	_, err := mustNew(t, &cli, Resolver(EnvResolver())).Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, 12, cli.Int)
+}