added parser struct

parse.go

@@ -1,8 +1,11 @@
 package arg
 
 import (
+	"errors"
 	"fmt"
+	"io"
 	"os"
+	"path/filepath"
 	"reflect"
 	"strconv"
 	"strings"
@@ -20,48 +23,74 @@ type spec struct {
 	wasPresent bool
 }
 
+// Parse returns this value to indicate that -h or --help were provided
+var ErrHelp = errors.New("help requested by user")
+
 // MustParse processes command line arguments and exits upon failure.
 func MustParse(dest ...interface{}) {
-	err := Parse(dest...)
+	p, err := NewParser(dest...)
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(1)
 	}
-}
-
-// Parse processes command line arguments and stores the result in args.
-func Parse(dest ...interface{}) error {
-	return ParseFrom(os.Args[1:], dest...)
-}
-
-// ParseFrom processes command line arguments and stores the result in args.
-func ParseFrom(args []string, dest ...interface{}) error {
-	// Add the help option if one is not already defined
-	var internal struct {
-		Help bool `arg:"-h,help:print this help message"`
+	err = p.Parse(os.Args[1:])
+	if err != nil {
+		fmt.Println(err)
+		writeUsage(os.Stdout, filepath.Base(os.Args[0]), p.spec)
+		os.Exit(1)
 	}
+}
 
-	// Parse the spec
-	dest = append(dest, &internal)
+// Parse processes command line arguments and stores them in dest.
+func Parse(dest ...interface{}) error {
+	p, err := NewParser(dest...)
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	return p.Parse(os.Args[1:])
+}
+
+// Parser represents a set of command line options with destination values
+type Parser struct {
+	spec []*spec
+}
+
+// NewParser constructs a parser from a list of destination structs
+func NewParser(dest ...interface{}) (*Parser, error) {
 	spec, err := extractSpec(dest...)
 	if err != nil {
-		return err
+		return nil, err
+	}
+	return &Parser{spec: spec}, nil
+}
+
+// Parse processes the given command line option, storing the results in the field
+// of the structs from which NewParser was constructed
+func (p *Parser) Parse(args []string) error {
+	// If -h or --help were specified then print usage
+	for _, arg := range args {
+		if arg == "-h" || arg == "--help" {
+			return ErrHelp
+		}
+		if arg == "--" {
+			break
+		}
 	}
 
-	// Process args
-	err = processArgs(spec, args)
+	// Process all command line arguments
+	err := process(p.spec, args)
 	if err != nil {
 		return err
 	}
 
-	// If -h or --help were specified then print help
-	if internal.Help {
-		writeUsage(os.Stdout, spec)
-		os.Exit(0)
-	}
-
 	// Validate
-	return validate(spec)
+	return validate(p.spec)
+}
+
+// WriteUsage writes usage information to the given writer
+func (p *Parser) WriteUsage(w io.Writer) {
+	writeUsage(w, filepath.Base(os.Args[0]), p.spec)
 }
 
 // extractSpec gets specifications for each argument from the tags in a struct
@@ -143,8 +172,9 @@ func extractSpec(dests ...interface{}) ([]*spec, error) {
 	return specs, nil
 }
 
-// processArgs processes arguments using a pre-constructed spec
-func processArgs(specs []*spec, args []string) error {
+// process goes through arguments the arguments one-by-one, parses them, and assigns the result to
+// the underlying struct field
+func process(specs []*spec, args []string) error {
 	// construct a map from --option to spec
 	optionMap := make(map[string]*spec)
 	for _, spec := range specs {

parse_test.go

@@ -8,15 +8,19 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func split(s string) []string {
-	return strings.Split(s, " ")
+func parse(cmdline string, dest interface{}) error {
+	p, err := NewParser(dest)
+	if err != nil {
+		return err
+	}
+	return p.Parse(strings.Split(cmdline, " "))
 }
 
 func TestStringSingle(t *testing.T) {
 	var args struct {
 		Foo string
 	}
-	err := ParseFrom(split("--foo bar"), &args)
+	err := parse("--foo bar", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "bar", args.Foo)
 }
@@ -30,7 +34,7 @@ func TestMixed(t *testing.T) {
 		Spam float32
 	}
 	args.Bar = 3
-	err := ParseFrom(split("123 -spam=1.2 -ham -f xyz"), &args)
+	err := parse("123 -spam=1.2 -ham -f xyz", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 	assert.Equal(t, 3, args.Bar)
@@ -43,7 +47,7 @@ func TestRequired(t *testing.T) {
 	var args struct {
 		Foo string `arg:"required"`
 	}
-	err := ParseFrom(nil, &args)
+	err := parse("", &args)
 	require.Error(t, err, "--foo is required")
 }
 
@@ -52,15 +56,15 @@ func TestShortFlag(t *testing.T) {
 		Foo string `arg:"-f"`
 	}
 
-	err := ParseFrom(split("-f xyz"), &args)
+	err := parse("-f xyz", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 
-	err = ParseFrom(split("-foo xyz"), &args)
+	err = parse("-foo xyz", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 
-	err = ParseFrom(split("--foo xyz"), &args)
+	err = parse("--foo xyz", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 }
@@ -71,7 +75,7 @@ func TestCaseSensitive(t *testing.T) {
 		Upper bool `arg:"-V"`
 	}
 
-	err := ParseFrom(split("-v"), &args)
+	err := parse("-v", &args)
 	require.NoError(t, err)
 	assert.True(t, args.Lower)
 	assert.False(t, args.Upper)
@@ -83,7 +87,7 @@ func TestCaseSensitive2(t *testing.T) {
 		Upper bool `arg:"-V"`
 	}
 
-	err := ParseFrom(split("-V"), &args)
+	err := parse("-V", &args)
 	require.NoError(t, err)
 	assert.False(t, args.Lower)
 	assert.True(t, args.Upper)
@@ -94,7 +98,7 @@ func TestPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional"`
 	}
-	err := ParseFrom(split("foo"), &args)
+	err := parse("foo", &args)
 	require.NoError(t, err)
 	assert.Equal(t, "foo", args.Input)
 	assert.Equal(t, "", args.Output)
@@ -105,7 +109,7 @@ func TestRequiredPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional,required"`
 	}
-	err := ParseFrom(split("foo"), &args)
+	err := parse("foo", &args)
 	assert.Error(t, err)
 }
 
@@ -114,7 +118,7 @@ func TestTooManyPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional"`
 	}
-	err := ParseFrom(split("foo bar baz"), &args)
+	err := parse("foo bar baz", &args)
 	assert.Error(t, err)
 }
 
@@ -123,7 +127,7 @@ func TestMultiple(t *testing.T) {
 		Foo []int
 		Bar []string
 	}
-	err := ParseFrom(split("--foo 1 2 3 --bar x y z"), &args)
+	err := parse("--foo 1 2 3 --bar x y z", &args)
 	require.NoError(t, err)
 	assert.Equal(t, []int{1, 2, 3}, args.Foo)
 	assert.Equal(t, []string{"x", "y", "z"}, args.Bar)

usage.go

@@ -4,11 +4,12 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"reflect"
 	"strings"
 )
 
-// Usage prints usage information to stdout information and exits with status zero
+// Usage prints usage information to stdout and exits with status zero
 func Usage(dest ...interface{}) {
 	if err := WriteUsage(os.Stdout, dest...); err != nil {
 		fmt.Println(err)
@@ -31,20 +32,12 @@ func WriteUsage(w io.Writer, dest ...interface{}) error {
 	if err != nil {
 		return err
 	}
-	writeUsage(w, spec)
+	writeUsage(w, filepath.Base(os.Args[0]), spec)
 	return nil
 }
 
-func synopsis(spec *spec, form string) string {
-	if spec.dest.Kind() == reflect.Bool {
-		return form
-	} else {
-		return form + " " + strings.ToUpper(spec.long)
-	}
-}
-
 // writeUsage writes usage information to the given writer
-func writeUsage(w io.Writer, specs []*spec) {
+func writeUsage(w io.Writer, cmd string, specs []*spec) {
 	var positionals, options []*spec
 	for _, spec := range specs {
 		if spec.positional {
@@ -54,7 +47,7 @@ func writeUsage(w io.Writer, specs []*spec) {
 		}
 	}
 
-	fmt.Fprint(w, "usage: ")
+	fmt.Fprint(w, "usage: %s ", cmd)
 
 	// write the option component of the one-line usage message
 	for _, spec := range options {
@@ -110,3 +103,11 @@ func writeUsage(w io.Writer, specs []*spec) {
 		}
 	}
 }
+
+func synopsis(spec *spec, form string) string {
+	if spec.dest.Kind() == reflect.Bool {
+		return form
+	} else {
+		return form + " " + strings.ToUpper(spec.long)
+	}
+}