Allow Mappers to contribute Vars to interpolation.

See #218.

kong.go

@@ -183,6 +183,9 @@ func (k *Kong) interpolateValue(value *Value, vars Vars) (err error) {
 	if len(value.Tag.Vars) > 0 {
 		vars = vars.CloneWith(value.Tag.Vars)
 	}
+	if varsContributor, ok := value.Mapper.(VarsContributor); ok {
+		vars = vars.CloneWith(varsContributor.Vars(value))
+	}
 	if value.Default, err = interpolate(value.Default, vars, nil); err != nil {
 		return fmt.Errorf("default value for %s: %s", value.Summary(), err)
 	}

mapper.go

@@ -115,6 +115,11 @@ type Mapper interface {
 	Decode(ctx *DecodeContext, target reflect.Value) error
 }
 
+// VarsContributor can be implemented by a Mapper to contribute Vars during interpolation.
+type VarsContributor interface {
+	Vars(ctx *Value) Vars
+}
+
 // A BoolMapper is a Mapper to a value that is a boolean.
 //
 // This is used solely for formatting help.

mapper_test.go

@@ -464,3 +464,34 @@ func (t testMapperWithPlaceHolder) Decode(ctx *kong.DecodeContext, target reflec
 func (t testMapperWithPlaceHolder) PlaceHolder(flag *kong.Flag) string {
 	return "/a/b/c"
 }
+
+func TestMapperVarsContributor(t *testing.T) {
+	var cli struct {
+		Flag string `help:"Some help with ${avar}"`
+	}
+	b := bytes.NewBuffer(nil)
+	k := mustNew(
+		t,
+		&cli,
+		kong.Writers(b, b),
+		kong.ValueMapper(&cli.Flag, testMapperVarsContributor{}),
+		kong.Exit(func(int) { panic("exit") }),
+	)
+	// Ensure that --help
+	require.Panics(t, func() {
+		_, err := k.Parse([]string{"--help"})
+		require.NoError(t, err)
+	})
+	require.Regexp(t, "--flag=STRING\\s+Some help with a var", b.String())
+}
+
+type testMapperVarsContributor struct{}
+
+func (t testMapperVarsContributor) Vars(value *kong.Value) kong.Vars {
+	return kong.Vars{"avar": "a var"}
+}
+
+func (t testMapperVarsContributor) Decode(ctx *kong.DecodeContext, target reflect.Value) error {
+	target.SetString("hi")
+	return nil
+}