add expected output for usage example

example_test.go

@@ -115,5 +115,23 @@ func Example_usageString() {
 		Dataset  string   `help:"dataset to use"`
 		Optimize int      `arg:"-O,help:optimization level"`
 	}
+
+	// This is only necessary when running inside golang's runnable example harness
+	osExit = func(int) {}
+
 	MustParse(&args)
+
+	// output:
+	// Usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]
+	//
+	// Positional arguments:
+	//   INPUT
+	//   OUTPUT
+	//
+	// Options:
+	//   --verbose, -v          verbosity level
+	//   --dataset DATASET      dataset to use
+	//   --optimize OPTIMIZE, -O OPTIMIZE
+	//                          optimization level
+	//   --help, -h             display this help and exit
 }

parse.go

@@ -13,6 +13,9 @@ import (
 	scalar "github.com/alexflint/go-scalar"
 )
 
+// to enable monkey-patching during tests
+var osExit = os.Exit
+
 // spec represents a command line option
 type spec struct {
 	dest       reflect.Value
@@ -38,20 +41,21 @@ func MustParse(dest ...interface{}) *Parser {
 	p, err := NewParser(Config{}, dest...)
 	if err != nil {
 		fmt.Println(err)
-		os.Exit(-1)
+		osExit(-1)
 	}
+
 	err = p.Parse(flags())
-	if err == ErrHelp {
+	switch {
+	case err == ErrHelp:
 		p.WriteHelp(os.Stdout)
-		os.Exit(0)
-	}
-	if err == ErrVersion {
+		osExit(0)
+	case err == ErrVersion:
 		fmt.Println(p.version)
-		os.Exit(0)
-	}
-	if err != nil {
+		osExit(0)
+	case err != nil:
 		p.Fail(err.Error())
 	}
+
 	return p
 }