separate help into WriteUsage and WriteHelp

README.md

@@ -19,17 +19,6 @@ $ ./example --foo=hello --bar
 hello true
 ```
 
-### Default values
-
-```go
-var args struct {
-	Foo string
-	Bar bool
-}
-args.Foo = "default value"
-arg.MustParse(&args)
-```
-
 ### Required arguments
 
 ```go
@@ -40,6 +29,12 @@ var args struct {
 arg.MustParse(&args)
 ```
 
+```shell
+$ ./example
+usage: example --foo FOO [--bar] 
+error: --foo is required
+```
+
 ### Positional arguments
 
 ```go
@@ -79,11 +74,22 @@ positional arguments:
   output
 
 options:
---verbose, -v            verbosity level
---dataset DATASET        dataset to use
---optimize OPTIMIZE, -O OPTIMIZE
-                         optimization level
---help, -h               print this help message
+  --verbose, -v            verbosity level
+  --dataset DATASET        dataset to use
+  --optimize OPTIMIZE, -O OPTIMIZE
+                           optimization level
+  --help, -h               print this help message
+```
+
+### Default values
+
+```go
+var args struct {
+	Foo string
+	Bar bool
+}
+args.Foo = "default value"
+arg.MustParse(&args)
 ```
 
 ### Arguments with multiple values

parse.go

@@ -64,11 +64,11 @@ func MustParse(dest ...interface{}) {
 	p, err := NewParser(dest...)
 	if err != nil {
 		fmt.Println(err)
-		os.Exit(1)
+		os.Exit(-1)
 	}
 	err = p.Parse(os.Args[1:])
 	if err == ErrHelp {
-		p.WriteUsage(os.Stdout)
+		p.WriteHelp(os.Stdout)
 		os.Exit(0)
 	}
 	if err != nil {

usage.go

@@ -11,9 +11,9 @@ import (
 
 // Fail prints usage information to stdout and exits with non-zero status
 func (p *Parser) Fail(msg string) {
-	fmt.Println(msg)
 	p.WriteUsage(os.Stdout)
-	os.Exit(1)
+	fmt.Println("error:", msg)
+	os.Exit(-1)
 }
 
 // WriteUsage writes usage information to the given writer
@@ -29,7 +29,7 @@ func (p *Parser) WriteUsage(w io.Writer) {
 
 	fmt.Fprintf(w, "usage: %s ", filepath.Base(os.Args[0]))
 
-	// write the option component of the one-line usage message
+	// write the option component of the usage message
 	for _, spec := range options {
 		if !spec.required {
 			fmt.Fprint(w, "[")
@@ -41,7 +41,7 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		fmt.Fprint(w, " ")
 	}
 
-	// write the positional component of the one-line usage message
+	// write the positional component of the usage message
 	for _, spec := range positionals {
 		up := strings.ToUpper(spec.long)
 		if spec.multiple {
@@ -52,6 +52,20 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		fmt.Fprint(w, " ")
 	}
 	fmt.Fprint(w, "\n")
+}
+
+// WriteHelp writes the usage string followed by the full help string for each option
+func (p *Parser) WriteHelp(w io.Writer) {
+	var positionals, options []*spec
+	for _, spec := range p.spec {
+		if spec.positional {
+			positionals = append(positionals, spec)
+		} else {
+			options = append(options, spec)
+		}
+	}
+
+	p.WriteUsage(w)
 
 	// write the list of positionals
 	if len(positionals) > 0 {
@@ -66,9 +80,9 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		fmt.Fprint(w, "\noptions:\n")
 		const colWidth = 25
 		for _, spec := range options {
-			left := fmt.Sprint(synopsis(spec, "--"+spec.long))
+			left := "  " + synopsis(spec, "--"+spec.long)
 			if spec.short != "" {
-				left += ", " + fmt.Sprint(synopsis(spec, "-"+spec.short))
+				left += ", " + synopsis(spec, "-"+spec.short)
 			}
 			fmt.Print(left)
 			if spec.help != "" {