add support for version string

3 files changed, 68 insertions, 11 deletions

diff --git a/parse.go b/parse.go
index b1193d0..f5fdd7f 100644
--- a/parse.go
+++ b/parse.go
@@ -27,6 +27,9 @@ type spec struct {
 // ErrHelp indicates that -h or --help were provided
 var ErrHelp = errors.New("help requested by user")
 
+// ErrVersion indicates that --version was provided
+var ErrVersion = errors.New("version requested by user")
+
 // MustParse processes command line arguments and exits upon failure
 func MustParse(dest ...interface{}) *Parser {
 	p, err := NewParser(Config{}, dest...)
@@ -39,6 +42,10 @@ func MustParse(dest ...interface{}) *Parser {
 		p.WriteHelp(os.Stdout)
 		os.Exit(0)
 	}
+	if err == ErrVersion {
+		fmt.Println(p.version)
+		os.Exit(0)
+	}
 	if err != nil {
 		p.Fail(err.Error())
 	}
@@ -61,14 +68,28 @@ type Config struct {
 
 // Parser represents a set of command line options with destination values
 type Parser struct {
-	spec   []*spec
-	config Config
+	spec    []*spec
+	config  Config
+	version string
+}
+
+// Versioned is the interface that the destination struct should implement to
+// make a version string appear at the top of the help message.
+type Versioned interface {
+	// Version returns the version string that will be printed on a line by itself
+	// at the top of the help message.
+	Version() string
 }
 
 // NewParser constructs a parser from a list of destination structs
 func NewParser(config Config, dests ...interface{}) (*Parser, error) {
-	var specs []*spec
+	p := Parser{
+		config: config,
+	}
 	for _, dest := range dests {
+		if dest, ok := dest.(Versioned); ok {
+			p.version = dest.Version()
+		}
 		v := reflect.ValueOf(dest)
 		if v.Kind() != reflect.Ptr {
 			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))
@@ -138,19 +159,16 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 					}
 				}
 			}
-			specs = append(specs, &spec)
+			p.spec = append(p.spec, &spec)
 		}
 	}
-	if config.Program == "" {
-		config.Program = "program"
+	if p.config.Program == "" {
+		p.config.Program = "program"
 		if len(os.Args) > 0 {
-			config.Program = filepath.Base(os.Args[0])
+			p.config.Program = filepath.Base(os.Args[0])
 		}
 	}
-	return &Parser{
-		spec:   specs,
-		config: config,
-	}, nil
+	return &p, nil
 }
 
 // Parse processes the given command line option, storing the results in the field
@@ -161,6 +179,9 @@ func (p *Parser) Parse(args []string) error {
 		if arg == "-h" || arg == "--help" {
 			return ErrHelp
 		}
+		if arg == "--version" {
+			return ErrVersion
+		}
 		if arg == "--" {
 			break
 		}
diff --git a/usage.go b/usage.go
index 2ee3953..f096f58 100644
--- a/usage.go
+++ b/usage.go
@@ -29,6 +29,10 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		}
 	}
 
+	if p.version != "" {
+		fmt.Fprintln(w, p.version)
+	}
+
 	fmt.Fprintf(w, "usage: %s", p.config.Program)
 
 	// write the option component of the usage message
@@ -97,6 +101,9 @@ func (p *Parser) WriteHelp(w io.Writer) {
 
 	// write the list of built in options
 	printOption(w, &spec{boolean: true, long: "help", short: "h", help: "display this help and exit"})
+	if p.version != "" {
+		printOption(w, &spec{boolean: true, long: "version", help: "display version and exit"})
+	}
 }
 
 func printOption(w io.Writer, spec *spec) {
diff --git a/usage_test.go b/usage_test.go
index b63a7d0..e60efdb 100644
--- a/usage_test.go
+++ b/usage_test.go
@@ -100,3 +100,32 @@ options:
 	p.WriteHelp(&help)
 	assert.Equal(t, expectedHelp, help.String())
 }
+
+type versioned struct{}
+
+// Version returns the version for this program
+func (versioned) Version() string {
+	return "example 3.2.1"
+}
+
+func TestUsageWithVersion(t *testing.T) {
+	expectedHelp := `example 3.2.1
+usage: example
+
+options:
+  --help, -h             display this help and exit
+  --version              display version and exit
+`
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &versioned{})
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	actual := help.String()
+	t.Logf("Expected:\n%s", expectedHelp)
+	t.Logf("Actual:\n%s", actual)
+	if expectedHelp != actual {
+		t.Fail()
+	}
+}