make error messages nicer

This changes the error message so that not the program help is printed
but instead the specific submenu help. This should result in much faster
and better understanding of the problematic situation.

flags.go

@@ -65,11 +65,12 @@ type (
   }
 
   Flags struct {
+    Name           string        // name of the sub function
     flagset        *flag.FlagSet // the flagset reference for printing the help
     flag_container *paramContainer
-    Flags          *flagSet    // the end result of the flag setting
+    Flags          *flagSet      // the end result of the flag setting
 
-    check_list     []flagCheck // list of all checks
+    check_list     []flagCheck   // list of all checks
   }
 
   flagCheck func()(error)
@@ -78,8 +79,9 @@ type (
 // create a new flag handler with the name of the subfunction
 func NewFlags(method_name string) *Flags {
   return &Flags{
+    Name:           method_name,
     Flags:          &flagSet{},
-    flagset:        flag.NewFlagSet(method_name, flag.ExitOnError),
+    flagset:        flag.NewFlagSet(method_name, flag.ContinueOnError),
     check_list:     make([]flagCheck, 0),
     flag_container: &paramContainer{},
   }
@@ -90,11 +92,21 @@ func (f *Flags) Parse(options []string) error {
   f.flagset.Parse(options)
   for _, check := range f.check_list {
     // TODO handle error in a betetr way (output specific help, not command help)
-    if err := check(); err != nil { return err }
+    if err := check(); err != nil {
+      f.Usagef("%s", err)
+      return err
+    }
   }
   return nil
 }
 
+func (f *Flags) Usagef(message string, args ...interface{}) {
+  fmt.Fprintf(os.Stderr, "error: " + message + "\n", args...)
+  fmt.Fprintf(os.Stderr, "usage: %s %s [options]\n", os.Args[0], f.Name)
+  fmt.Fprint(os.Stderr,  "where options are:\n")
+  f.flagset.PrintDefaults()
+}
+
 // add the private key option to the requested flags
 func (f *Flags) AddPrivateKey() {
   f.check_list = append(f.check_list, f.parsePrivateKey)

main.go

@@ -48,18 +48,18 @@ func create_private_key() {
   fs.AddOutput()
   fs.AddPrivateKeyGenerationFlags()
   err := fs.Parse(program_args())
-  if err != nil { crash_with_help(1, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
 
   var pk pki.Pemmer
   switch fs.Flags.PrivateKeyGenerationFlags.Type {
     case "ecdsa": pk, err = pki.NewPrivateKeyEcdsa(fs.Flags.PrivateKeyGenerationFlags.Curve)
     case "rsa":   pk, err = pki.NewPrivateKeyRsa(fs.Flags.PrivateKeyGenerationFlags.Size)
   }
-  if err != nil { crash_with_help(2, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
   marsh_pem, err := pk.MarshalPem()
-  if err != nil { crash_with_help(2, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
   _, err = marsh_pem.WriteTo(fs.Flags.Output)
-  if err != nil { crash_with_help(2, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
 }
 
 // create a public key derived from a private key
@@ -68,14 +68,14 @@ func create_public_key() {
   fs.AddPrivateKey()
   fs.AddOutput()
   err := fs.Parse(program_args())
-  if err != nil { crash_with_help(1, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
 
   var pub_key pki.Pemmer
   pub_key = fs.Flags.PrivateKey.Public()
   marsh_pem, err := pub_key.MarshalPem()
-  if err != nil { crash_with_help(2, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
   _, err = marsh_pem.WriteTo(fs.Flags.Output)
-  if err != nil { crash_with_help(2, fmt.Sprintf("%s", err)) }
+  if err != nil { os.Exit(2) }
 }
 
 // print the module help