feat: support optionally passing kong.Context to Validate()

Fixes #340

README.md

@@ -663,15 +663,20 @@ func main() {
 ## Validation
 
 Kong does validation on the structure of a command-line, but also supports
-extensible validation. Any node in the tree may implement the following
-interface:
+extensible validation. Any node in the tree may implement either of the following interfaces:
 
 ```go
 type Validatable interface {
     Validate() error
  }
 ```
 
+```go
+type Validatable interface {
+    Validate(kctx *kong.Context) error
+ }
+```
+
 If one of these nodes is in the active command-line it will be called during
 normal validation.
 

context.go

@@ -208,7 +208,7 @@ func (c *Context) Validate() error { //nolint: gocyclo
 			desc = node.Path()
 		}
 		if validate := isValidatable(value); validate != nil {
-			if err := validate.Validate(); err != nil {
+			if err := validate.Validate(c); err != nil {
 				if desc != "" {
 					return fmt.Errorf("%s: %w", desc, err)
 				}
@@ -1094,12 +1094,23 @@ func findPotentialCandidates(needle string, haystack []string, format string, ar
 }
 
 type validatable interface{ Validate() error }
+type extendedValidatable interface {
+	Validate(kctx *Context) error
+}
+
+// Proxy a validatable function to the extendedValidatable interface
+type validatableFunc func() error
 
-func isValidatable(v reflect.Value) validatable {
+func (f validatableFunc) Validate(kctx *Context) error { return f() }
+
+func isValidatable(v reflect.Value) extendedValidatable {
 	if !v.IsValid() || (v.Kind() == reflect.Ptr || v.Kind() == reflect.Slice || v.Kind() == reflect.Map) && v.IsNil() {
 		return nil
 	}
 	if validate, ok := v.Interface().(validatable); ok {
+		return validatableFunc(validate.Validate)
+	}
+	if validate, ok := v.Interface().(extendedValidatable); ok {
 		return validate
 	}
 	if v.CanAddr() {

kong_test.go

@@ -1466,6 +1466,19 @@ func TestValidateArg(t *testing.T) {
 	assert.EqualError(t, err, "<arg>: flag error")
 }
 
+type extendedValidateFlag string
+
+func (v *extendedValidateFlag) Validate(kctx *kong.Context) error { return errors.New("flag error") }
+
+func TestExtendedValidateFlag(t *testing.T) {
+	cli := struct {
+		Flag extendedValidateFlag
+	}{}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--flag=one"})
+	assert.EqualError(t, err, "--flag: flag error")
+}
+
 func TestPointers(t *testing.T) {
 	cli := struct {
 		Mapped *mappedValue