move more stuff over to the parser struct

parse.go

@@ -3,9 +3,7 @@ package arg
 import (
 	"errors"
 	"fmt"
-	"io"
 	"os"
-	"path/filepath"
 	"reflect"
 	"strconv"
 	"strings"
@@ -23,10 +21,10 @@ type spec struct {
 	wasPresent bool
 }
 
-// Parse returns this value to indicate that -h or --help were provided
+// ErrHelp indicates that -h or --help were provided
 var ErrHelp = errors.New("help requested by user")
 
-// MustParse processes command line arguments and exits upon failure.
+// MustParse processes command line arguments and exits upon failure
 func MustParse(dest ...interface{}) {
 	p, err := NewParser(dest...)
 	if err != nil {
@@ -34,19 +32,20 @@ func MustParse(dest ...interface{}) {
 		os.Exit(1)
 	}
 	err = p.Parse(os.Args[1:])
+	if err == ErrHelp {
+		p.WriteUsage(os.Stdout)
+		os.Exit(0)
+	}
 	if err != nil {
-		fmt.Println(err)
-		writeUsage(os.Stdout, filepath.Base(os.Args[0]), p.spec)
-		os.Exit(1)
+		p.Fail(err.Error())
 	}
 }
 
-// Parse processes command line arguments and stores them in dest.
+// 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 err
 	}
 	return p.Parse(os.Args[1:])
 }
@@ -57,44 +56,7 @@ type Parser struct {
 }
 
 // NewParser constructs a parser from a list of destination structs
-func NewParser(dest ...interface{}) (*Parser, error) {
-	spec, err := extractSpec(dest...)
-	if err != nil {
-		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 all command line arguments
-	err := process(p.spec, args)
-	if err != nil {
-		return err
-	}
-
-	// Validate
-	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
-func extractSpec(dests ...interface{}) ([]*spec, error) {
+func NewParser(dests ...interface{}) (*Parser, error) {
 	var specs []*spec
 	for _, dest := range dests {
 		v := reflect.ValueOf(dest)
@@ -169,10 +131,33 @@ func extractSpec(dests ...interface{}) ([]*spec, error) {
 			specs = append(specs, &spec)
 		}
 	}
-	return specs, nil
+	return &Parser{spec: specs}, nil
 }
 
-// process goes through arguments the arguments one-by-one, parses them, and assigns the result to
+// 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 all command line arguments
+	err := process(p.spec, args)
+	if err != nil {
+		return err
+	}
+
+	// Validate
+	return validate(p.spec)
+}
+
+// process goes through 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

usage.go

@@ -9,37 +9,17 @@ import (
 	"strings"
 )
 
-// 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)
-	}
-	os.Exit(0)
-}
-
 // Fail prints usage information to stdout and exits with non-zero status
-func Fail(msg string, dest ...interface{}) {
+func (p *Parser) Fail(msg string) {
 	fmt.Println(msg)
-	if err := WriteUsage(os.Stdout, dest...); err != nil {
-		fmt.Println(err)
-	}
+	p.WriteUsage(os.Stdout)
 	os.Exit(1)
 }
 
 // WriteUsage writes usage information to the given writer
-func WriteUsage(w io.Writer, dest ...interface{}) error {
-	spec, err := extractSpec(dest...)
-	if err != nil {
-		return err
-	}
-	writeUsage(w, filepath.Base(os.Args[0]), spec)
-	return nil
-}
-
-// writeUsage writes usage information to the given writer
-func writeUsage(w io.Writer, cmd string, specs []*spec) {
+func (p *Parser) WriteUsage(w io.Writer) {
 	var positionals, options []*spec
-	for _, spec := range specs {
+	for _, spec := range p.spec {
 		if spec.positional {
 			positionals = append(positionals, spec)
 		} else {
@@ -47,7 +27,7 @@ func writeUsage(w io.Writer, cmd string, specs []*spec) {
 		}
 	}
 
-	fmt.Fprint(w, "usage: %s ", cmd)
+	fmt.Fprintf(w, "usage: %s ", filepath.Base(os.Args[0]))
 
 	// write the option component of the one-line usage message
 	for _, spec := range options {