BeforeValidate hook

README.md

@@ -303,10 +303,8 @@ func main() {
 
 ## Hooks: BeforeReset(), BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option
 
-If a node in the grammar has a `BeforeReset(...)`, `BeforeResolve
-(...)`, `BeforeApply(...) error` and/or `AfterApply(...) error` method, those
-methods will be called before values are reset, before validation/assignment,
-and after validation/assignment, respectively.
+If a node in the grammar has a `BeforeReset(...)`, `BeforeResolve(...)`, `BeforeApply(...) error`, `AfterApply(...) error`, and/or `BeforeValidate(...) error` method, 
+those methods will be called before values are reset, before validation/assignment, and after validation/assignment, respectively.
 
 The `--help` flag is implemented with a `BeforeReset` hook.
 

hooks.go

@@ -12,6 +12,12 @@ type BeforeApply interface {
 	BeforeApply(args ...interface{}) error
 }
 
+// BeforeValidate is a documentation-only interface describing hooks that run before validation is applied.
+type BeforeValidate interface {
+	// This is not the correct signature - see README for details.
+	BeforeValidate(args ...interface{}) error
+}
+
 // AfterApply is a documentation-only interface describing hooks that run after values are set.
 type AfterApply interface {
 	// This is not the correct signature - see README for details.

kong.go

@@ -302,6 +302,9 @@ func (k *Kong) Parse(args []string) (ctx *Context, err error) {
 	if _, err = ctx.Apply(); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
+	if err = k.applyHook(ctx, "BeforeValidate"); err != nil {
+		return nil, &ParseError{error: err, Context: ctx}
+	}
 	if err = ctx.Validate(); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}

kong_test.go

@@ -455,6 +455,12 @@ func (h *hookValue) BeforeApply(ctx *hookContext) error {
 	return nil
 }
 
+func (h *hookValue) BeforeValidate(ctx *hookContext) error {
+	*h = hookValue(strings.ToUpper(string(*h)))
+	ctx.values = append(ctx.values, "beforeValidate:"+string(*h))
+	return nil
+}
+
 func (h *hookValue) AfterApply(ctx *hookContext) error {
 	ctx.values = append(ctx.values, "after:"+string(*h))
 	return nil
@@ -482,9 +488,9 @@ func TestHooks(t *testing.T) {
 		values hookContext
 	}{
 		{"Command", "one", hookContext{true, nil}},
-		{"Arg", "one two", hookContext{true, []string{"before:", "after:two"}}},
-		{"Flag", "one --three=THREE", hookContext{true, []string{"before:", "after:THREE"}}},
-		{"ArgAndFlag", "one two --three=THREE", hookContext{true, []string{"before:", "before:", "after:two", "after:THREE"}}},
+		{"Arg", "one two", hookContext{true, []string{"before:", "beforeValidate:TWO", "after:TWO"}}},
+		{"Flag", "one --three=THREE", hookContext{true, []string{"before:", "beforeValidate:THREE", "after:THREE"}}},
+		{"ArgAndFlag", "one two --three=THREE", hookContext{true, []string{"before:", "before:", "beforeValidate:TWO", "beforeValidate:THREE", "after:TWO", "after:THREE"}}},
 	}
 
 	var cli struct {
@@ -776,8 +782,8 @@ func TestHooksCalledForDefault(t *testing.T) {
 	ctx := &hookContext{}
 	_, err := mustNew(t, &cli, kong.Bind(ctx)).Parse(nil)
 	assert.NoError(t, err)
-	assert.Equal(t, "default", string(cli.Flag))
-	assert.Equal(t, []string{"before:default", "after:default"}, ctx.values)
+	assert.Equal(t, "DEFAULT", string(cli.Flag))
+	assert.Equal(t, []string{"before:default", "beforeValidate:DEFAULT", "after:DEFAULT"}, ctx.values)
 }
 
 func TestEnum(t *testing.T) {

resolver_test.go

@@ -368,8 +368,8 @@ func TestResolverTriggersHooks(t *testing.T) {
 	_, err := mustNew(t, &cli, kong.Bind(ctx), kong.Resolvers(first)).Parse(nil)
 	assert.NoError(t, err)
 
-	assert.Equal(t, "one", string(cli.Flag))
-	assert.Equal(t, []string{"before:", "after:one"}, ctx.values)
+	assert.Equal(t, "ONE", string(cli.Flag))
+	assert.Equal(t, []string{"before:", "beforeValidate:ONE", "after:ONE"}, ctx.values)
 }
 
 type validatingResolver struct {