Apply hooks to default values.

build.go

@@ -180,6 +180,7 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 		Name:         name,
 		Help:         tag.Help,
 		Default:      tag.Default,
+		DefaultValue: reflect.New(fv.Type()).Elem(),
 		Mapper:       mapper,
 		Tag:          tag,
 		Target:       fv,
@@ -190,6 +191,13 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 		Format:   tag.Format,
 	}
 
+	if value.Default != "" {
+		err := value.Parse(Scan(tag.Default), value.DefaultValue)
+		if err != nil {
+			fail("invalid default value %q for field type %s.%s (of type %s)", value.Default, v.Type(), ft.Name, ft.Type)
+		}
+	}
+
 	if tag.Arg {
 		node.Positional = append(node.Positional, value)
 	} else {

context.go

@@ -181,18 +181,18 @@ func (c *Context) AddResolver(resolver ResolverFunc) {
 	c.resolvers = append(c.resolvers, resolver)
 }
 
-// FlagValue returns the set value of a flag if it was encountered and exists.
+// FlagValue returns the set value of a flag if it was encountered and exists, or its default value.
 func (c *Context) FlagValue(flag *Flag) interface{} {
 	for _, trace := range c.Path {
 		if trace.Flag == flag {
 			v, ok := c.values[trace.Flag.Value]
 			if !ok {
-				return nil
+				break
 			}
 			return v.Interface()
 		}
 	}
-	return nil
+	return flag.DefaultValue.Interface()
 }
 
 // Recursively reset values to defaults (as specified in the grammar) or the zero value.

help.go

@@ -13,6 +13,20 @@ const (
 	defaultColumnPadding = 4
 )
 
+// Help flag.
+type helpValue bool
+
+func (h helpValue) BeforeApply(ctx *Context) error {
+	options := ctx.Kong.helpOptions
+	options.Summary = false
+	err := ctx.Kong.help(options, ctx)
+	if err != nil {
+		return err
+	}
+	ctx.Kong.Exit(1)
+	return nil
+}
+
 // HelpOptions for HelpPrinters.
 type HelpOptions struct {
 	// Don't print top-level usage summary.

kong.go

@@ -151,19 +151,6 @@ func (k *Kong) interpolateValue(value *Value, vars Vars) (err error) {
 	return nil
 }
 
-type helpValue bool
-
-func (h helpValue) BeforeApply(ctx *Context) error {
-	options := ctx.Kong.helpOptions
-	options.Summary = false
-	err := ctx.Kong.help(options, ctx)
-	if err != nil {
-		return err
-	}
-	ctx.Kong.Exit(1)
-	return nil
-}
-
 // Provide additional builtin flags, if any.
 func (k *Kong) extraFlags() []*Flag {
 	if k.noDefaultHelp {
@@ -248,6 +235,35 @@ func (k *Kong) applyHook(ctx *Context, name string) error {
 			return err
 		}
 	}
+	// Path[0] will always be the app root.
+	return k.applyHookToDefaultFlags(ctx, ctx.Path[0].Node(), name)
+}
+
+// Call hook on any unset flags with default values.
+func (k *Kong) applyHookToDefaultFlags(ctx *Context, node *Node, name string) error {
+	if node == nil {
+		return nil
+	}
+	bindings := k.bindings.clone().add(ctx).add(node.Vars().CloneWith(k.vars))
+	for _, flag := range node.Flags {
+		if flag.Default == "" || ctx.values[flag.Value].IsValid() || !flag.Target.IsValid() {
+			continue
+		}
+		method := getMethod(flag.Target, name)
+		if !method.IsValid() {
+			continue
+		}
+		path := &Path{Flag: flag}
+		if err := callMethod(name, flag.Target, method, bindings.clone().add(path)); err != nil {
+			return err
+		}
+	}
+	for _, branch := range node.Children {
+		err := k.applyHookToDefaultFlags(ctx, branch, name)
+		if err != nil {
+			return err
+		}
+	}
 	return nil
 }
 

kong_test.go

@@ -642,3 +642,15 @@ func TestUnnamedFieldEmbeds(t *testing.T) {
 	require.Contains(t, buf.String(), `--one-flag=STRING`)
 	require.Contains(t, buf.String(), `--two-flag=STRING`)
 }
+
+func TestHooksCalledForDefault(t *testing.T) {
+	var cli struct {
+		Flag hookValue `default:"default"`
+	}
+
+	ctx := &hookContext{}
+	_, err := mustNew(t, &cli, kong.Bind(ctx)).Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "default", string(cli.Flag))
+	require.Equal(t, []string{"before:", "after:default"}, ctx.values)
+}

model.go

@@ -200,6 +200,7 @@ type Value struct {
 	Name         string
 	Help         string
 	Default      string
+	DefaultValue reflect.Value
 	Enum         string
 	Mapper       Mapper
 	Tag          *Tag

resolver_test.go

@@ -208,3 +208,24 @@ func TestResolverSatisfiesRequired(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 1, cli.Int)
 }
+
+func TestResolverTriggersHooks(t *testing.T) {
+	ctx := &hookContext{}
+
+	var cli struct {
+		Flag hookValue
+	}
+
+	var first kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+		if flag.Name == "flag" {
+			return "1", nil
+		}
+		return "", nil
+	}
+
+	_, err := mustNew(t, &cli, kong.Bind(ctx), kong.Resolver(first)).Parse(nil)
+	require.NoError(t, err)
+
+	require.Equal(t, "1", string(cli.Flag))
+	require.Equal(t, []string{"before:", "after:1"}, ctx.values)
+}