Indent multi-line messages/errors.

.golangci.yml

@@ -31,3 +31,4 @@ issues:
     # Very commonly not checked.
     - 'Error return value of .(.*\.Help|.*\.MarkFlagRequired|(os\.)?std(out|err)\..*|.*Close|.*Flush|os\.Remove(All)?|.*printf?|os\.(Un)?Setenv). is not checked'
     - 'exported method (.*\.MarshalJSON|.*\.UnmarshalJSON) should have comment or be unexported'
+    - 'composite literal uses unkeyed fields'

build.go

@@ -49,7 +49,7 @@ func flattenedFields(v reflect.Value) (out []flattenedField) {
 		if !fv.CanSet() {
 			continue
 		}
-		out = append(out, flattenedField{ft, fv})
+		out = append(out, flattenedField{field: ft, value: fv})
 	}
 	return
 }

kong.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"strings"
 )
 
 // Error reported by Kong.
@@ -14,7 +15,7 @@ type Error struct{ msg string }
 func (e Error) Error() string { return e.msg }
 
 func fail(format string, args ...interface{}) {
-	panic(Error{fmt.Sprintf(format, args...)})
+	panic(Error{msg: fmt.Sprintf(format, args...)})
 }
 
 // Must creates a new Parser or panics if there is an error.
@@ -170,15 +171,23 @@ func (k *Kong) applyHooks(ctx *Context) error {
 	return nil
 }
 
+func formatMultilineMessage(w io.Writer, leader string, format string, args ...interface{}) {
+	lines := strings.Split(fmt.Sprintf(format, args...), "\n")
+	fmt.Fprintf(w, "%s%s\n", leader, lines[0])
+	for _, line := range lines[1:] {
+		fmt.Fprintf(w, "%*s%s\n", len(leader), " ", line)
+	}
+}
+
 // Printf writes a message to Kong.Stdout with the application name prefixed.
 func (k *Kong) Printf(format string, args ...interface{}) *Kong {
-	fmt.Fprintf(k.Stdout, k.Model.Name+": "+format, args...)
+	formatMultilineMessage(k.Stdout, k.Model.Name+": ", format, args...)
 	return k
 }
 
 // Errorf writes a message to Kong.Stderr with the application name prefixed.
 func (k *Kong) Errorf(format string, args ...interface{}) *Kong {
-	fmt.Fprintf(k.Stderr, k.Model.Name+": error: "+format, args...)
+	formatMultilineMessage(k.Stderr, k.Model.Name+": error: ", format, args...)
 	return k
 }
 
@@ -191,7 +200,7 @@ func (k *Kong) FatalIfErrorf(err error, args ...interface{}) {
 	if len(args) > 0 {
 		msg = fmt.Sprintf(args[0].(string), args[1:]...) + ": " + err.Error()
 	}
-	k.Errorf("%s\n", msg)
+	k.Errorf("%s", msg)
 	k.Exit(1)
 }
 

kong_test.go

@@ -1,6 +1,7 @@
 package kong
 
 import (
+	"bytes"
 	"strings"
 	"testing"
 
@@ -10,6 +11,7 @@ import (
 func mustNew(t *testing.T, cli interface{}, options ...Option) *Kong {
 	t.Helper()
 	options = append([]Option{
+		Name("test"),
 		Exit(func(int) {
 			t.Helper()
 			t.Fatalf("unexpected exit()")
@@ -444,3 +446,11 @@ func TestSliceWithDisabledSeparator(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []string{"a,b", "b,c"}, cli.Flag)
 }
+
+func TestMultilineMessage(t *testing.T) {
+	w := &bytes.Buffer{}
+	var cli struct{}
+	p := mustNew(t, &cli, Writers(w, w))
+	p.Printf("hello\nworld")
+	require.Equal(t, "test: hello\n      world\n", w.String())
+}