Decoders are now field mappers.

Mappers are responsible for mapping from command-line input to Go. This
is typically just decoding, but also includes other information such as
if the field is a bool.

mapper_test.go

@@ -0,0 +1,42 @@
+package kong
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestValueMapper(t *testing.T) {
+	var cli struct {
+		Flag string
+	}
+	k := mustNew(t, &cli, ValueMapper(&cli.Flag, testMooMapper{}))
+	_, err := k.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "", cli.Flag)
+	_, err = k.Parse([]string{"--flag"})
+	require.NoError(t, err)
+	require.Equal(t, "MOO", cli.Flag)
+}
+
+func TestNamedMapper(t *testing.T) {
+	var cli struct {
+		Flag string `type:"moo"`
+	}
+	k := mustNew(t, &cli, NamedMapper("moo", testMooMapper{}))
+	_, err := k.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "", cli.Flag)
+	_, err = k.Parse([]string{"--flag"})
+	require.NoError(t, err)
+	require.Equal(t, "MOO", cli.Flag)
+}
+
+type testMooMapper struct{}
+
+func (testMooMapper) Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	target.SetString("MOO")
+	return nil
+}
+func (testMooMapper) IsBool() bool { return true }