make it possible to override the name of the program

parse.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"reflect"
 	"strconv"
 	"strings"
@@ -27,7 +28,7 @@ var ErrHelp = errors.New("help requested by user")
 
 // MustParse processes command line arguments and exits upon failure
 func MustParse(dest ...interface{}) *Parser {
-	p, err := NewParser(dest...)
+	p, err := NewParser(Config{}, dest...)
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(-1)
@@ -45,20 +46,26 @@ func MustParse(dest ...interface{}) *Parser {
 
 // Parse processes command line arguments and stores them in dest
 func Parse(dest ...interface{}) error {
-	p, err := NewParser(dest...)
+	p, err := NewParser(Config{}, dest...)
 	if err != nil {
 		return err
 	}
 	return p.Parse(os.Args[1:])
 }
 
+// Config represents configuration options for an argument parser
+type Config struct {
+	Program string // Program is the name of the program used in the help text
+}
+
 // Parser represents a set of command line options with destination values
 type Parser struct {
-	spec []*spec
+	spec   []*spec
+	config Config
 }
 
 // NewParser constructs a parser from a list of destination structs
-func NewParser(dests ...interface{}) (*Parser, error) {
+func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 	var specs []*spec
 	for _, dest := range dests {
 		v := reflect.ValueOf(dest)
@@ -138,7 +145,16 @@ func NewParser(dests ...interface{}) (*Parser, error) {
 			specs = append(specs, &spec)
 		}
 	}
-	return &Parser{spec: specs}, nil
+	if config.Program == "" {
+		config.Program = "program"
+		if len(os.Args) > 0 {
+			config.Program = filepath.Base(os.Args[0])
+		}
+	}
+	return &Parser{
+		spec:   specs,
+		config: config,
+	}, nil
 }
 
 // Parse processes the given command line option, storing the results in the field

parse_test.go

@@ -10,7 +10,7 @@ import (
 )
 
 func parse(cmdline string, dest interface{}) error {
-	p, err := NewParser(dest)
+	p, err := NewParser(Config{}, dest)
 	if err != nil {
 		return err
 	}

usage.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path/filepath"
 	"reflect"
 	"strings"
 )
@@ -30,7 +29,7 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		}
 	}
 
-	fmt.Fprintf(w, "usage: %s", filepath.Base(os.Args[0]))
+	fmt.Fprintf(w, "usage: %s", p.config.Program)
 
 	// write the option component of the usage message
 	for _, spec := range options {
@@ -114,8 +113,7 @@ func printOption(w io.Writer, spec *spec) {
 		}
 		fmt.Fprint(w, spec.help)
 	}
-	// Check if spec.dest is zero value or not
-	// If it isn't a default value have been added
+	// If spec.dest is not the zero value then a default value has been added.
 	v := spec.dest
 	if v.IsValid() {
 		z := reflect.Zero(v.Type())

usage_test.go

@@ -40,7 +40,7 @@ options:
 	}
 	args.Name = "Foo Bar"
 	args.Value = 42
-	p, err := NewParser(&args)
+	p, err := NewParser(Config{}, &args)
 	require.NoError(t, err)
 
 	os.Args[0] = "example"
@@ -68,7 +68,25 @@ options:
 		VeryLongPositionalWithHelp string `arg:"positional,help:this positional argument is very long"`
 	}
 
-	p, err := NewParser(&args)
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	os.Args[0] = "example"
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp, help.String())
+}
+
+func TestUsageWithProgramName(t *testing.T) {
+	expectedHelp := `usage: myprogram
+
+options:
+  --help, -h             display this help and exit
+`
+	config := Config{
+		Program: "myprogram",
+	}
+	p, err := NewParser(config, &struct{}{})
 	require.NoError(t, err)
 
 	os.Args[0] = "example"