added usage generation

parse.go

@@ -1,56 +1,16 @@
-package arguments
+package arg
 
 import (
 	"fmt"
-	"log"
 	"os"
 	"reflect"
 	"strconv"
 	"strings"
 )
 
-// MustParse processes command line arguments and exits upon failure.
+// spec represents a command line option
-func MustParse(dest interface{}) {
-	err := Parse(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(dest, os.Args)
-}
-
-// ParseFrom processes command line arguments and stores the result in args.
-func ParseFrom(dest interface{}, args []string) error {
-	v := reflect.ValueOf(dest)
-	if v.Kind() != reflect.Ptr {
-		panic(fmt.Sprintf("%s is not a pointer type", v.Type().Name()))
-	}
-	v = v.Elem()
-
-	// Parse the spec
-	spec, err := extractSpec(v.Type())
-	if err != nil {
-		return err
-	}
-
-	// Process args
-	err = processArgs(v, spec, args)
-	if err != nil {
-		return err
-	}
-
-	// Validate
-	return validate(spec)
-}
-
-// spec represents information about an argument extracted from struct tags
 type spec struct {
-	field      reflect.StructField
+	dest       reflect.Value
-	index      int
 	long       string
 	short      string
 	multiple   bool
@@ -60,13 +20,64 @@ type spec struct {
 	wasPresent bool
 }
 
-// extractSpec gets specifications for each argument from the tags in a struct
+// MustParse processes command line arguments and exits upon failure.
-func extractSpec(t reflect.Type) ([]*spec, error) {
+func MustParse(dest ...interface{}) {
-	if t.Kind() != reflect.Struct {
+	err := Parse(dest...)
-		panic(fmt.Sprintf("%s is not a struct pointer", t.Name()))
+	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"`
 	}
 
+	// Parse the spec
+	dest = append(dest, &internal)
+	spec, err := extractSpec(dest...)
+	if err != nil {
+		return err
+	}
+
+	// Process args
+	err = processArgs(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)
+}
+
+// extractSpec gets specifications for each argument from the tags in a struct
+func extractSpec(dests ...interface{}) ([]*spec, error) {
 	var specs []*spec
+	for _, dest := range dests {
+		v := reflect.ValueOf(dest)
+		if v.Kind() != reflect.Ptr {
+			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))
+		}
+		v = v.Elem()
+		if v.Kind() != reflect.Struct {
+			panic(fmt.Sprintf("%T is not a struct pointer", dest))
+		}
+
+		t := v.Type()
 		for i := 0; i < t.NumField(); i++ {
 			// Check for the ignore switch in the tag
 			field := t.Field(i)
@@ -77,13 +88,11 @@ func extractSpec(t reflect.Type) ([]*spec, error) {
 
 			spec := spec{
 				long: strings.ToLower(field.Name),
-			field: field,
+				dest: v.Field(i),
-			index: i,
 			}
 
 			// Get the scalar type for this field
 			scalarType := field.Type
-		log.Println(field.Name, field.Type, field.Type.Kind())
 			if scalarType.Kind() == reflect.Slice {
 				spec.multiple = true
 				scalarType = scalarType.Elem()
@@ -130,11 +139,12 @@ func extractSpec(t reflect.Type) ([]*spec, error) {
 			}
 			specs = append(specs, &spec)
 		}
+	}
 	return specs, nil
 }
 
 // processArgs processes arguments using a pre-constructed spec
-func processArgs(dest reflect.Value, specs []*spec, args []string) error {
+func processArgs(specs []*spec, args []string) error {
 	// construct a map from --option to spec
 	optionMap := make(map[string]*spec)
 	for _, spec := range specs {
@@ -192,7 +202,7 @@ func processArgs(dest reflect.Value, specs []*spec, args []string) error {
 			} else {
 				values = append(values, value)
 			}
-			err := setSlice(dest.Field(spec.index), values)
+			err := setSlice(spec.dest, values)
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", arg, err)
 			}
@@ -200,7 +210,7 @@ func processArgs(dest reflect.Value, specs []*spec, args []string) error {
 		}
 
 		// if it's a flag and it has no value then set the value to true
-		if spec.field.Type.Kind() == reflect.Bool && value == "" {
+		if spec.dest.Kind() == reflect.Bool && value == "" {
 			value = "true"
 		}
 
@@ -213,7 +223,7 @@ func processArgs(dest reflect.Value, specs []*spec, args []string) error {
 			i++
 		}
 
-		err := setScalar(dest.Field(spec.index), value)
+		err := setScalar(spec.dest, value)
 		if err != nil {
 			return fmt.Errorf("error processing %s: %v", arg, err)
 		}
@@ -221,22 +231,21 @@ func processArgs(dest reflect.Value, specs []*spec, args []string) error {
 
 	// process positionals
 	for _, spec := range specs {
-		label := strings.ToLower(spec.field.Name)
 		if spec.positional {
 			if spec.multiple {
-				err := setSlice(dest.Field(spec.index), positionals)
+				err := setSlice(spec.dest, positionals)
 				if err != nil {
-					return fmt.Errorf("error processing %s: %v", label, err)
+					return fmt.Errorf("error processing %s: %v", spec.long, err)
 				}
 				positionals = nil
 			} else if len(positionals) > 0 {
-				err := setScalar(dest.Field(spec.index), positionals[0])
+				err := setScalar(spec.dest, positionals[0])
 				if err != nil {
-					return fmt.Errorf("error processing %s: %v", label, err)
+					return fmt.Errorf("error processing %s: %v", spec.long, err)
 				}
 				positionals = positionals[1:]
 			} else if spec.required {
-				return fmt.Errorf("%s is required", label)
+				return fmt.Errorf("%s is required", spec.long)
 			}
 		}
 	}
@@ -250,7 +259,7 @@ func processArgs(dest reflect.Value, specs []*spec, args []string) error {
 func validate(spec []*spec) error {
 	for _, arg := range spec {
 		if !arg.positional && arg.required && !arg.wasPresent {
-			return fmt.Errorf("--%s is required", strings.ToLower(arg.field.Name))
+			return fmt.Errorf("--%s is required", arg.long)
 		}
 	}
 	return nil

parse_test.go

@@ -1,4 +1,4 @@
-package arguments
+package arg
 
 import (
 	"strings"
@@ -16,7 +16,7 @@ func TestStringSingle(t *testing.T) {
 	var args struct {
 		Foo string
 	}
-	err := ParseFrom(&args, split("--foo bar"))
+	err := ParseFrom(split("--foo bar"), &args)
 	require.NoError(t, err)
 	assert.Equal(t, "bar", args.Foo)
 }
@@ -30,7 +30,7 @@ func TestMixed(t *testing.T) {
 		Spam float32
 	}
 	args.Bar = 3
-	err := ParseFrom(&args, split("123 -spam=1.2 -ham -f xyz"))
+	err := ParseFrom(split("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 +43,7 @@ func TestRequired(t *testing.T) {
 	var args struct {
 		Foo string `arg:"required"`
 	}
-	err := ParseFrom(&args, nil)
+	err := ParseFrom(nil, &args)
 	require.Error(t, err, "--foo is required")
 }
 
@@ -52,15 +52,15 @@ func TestShortFlag(t *testing.T) {
 		Foo string `arg:"-f"`
 	}
 
-	err := ParseFrom(&args, split("-f xyz"))
+	err := ParseFrom(split("-f xyz"), &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 
-	err = ParseFrom(&args, split("-foo xyz"))
+	err = ParseFrom(split("-foo xyz"), &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 
-	err = ParseFrom(&args, split("--foo xyz"))
+	err = ParseFrom(split("--foo xyz"), &args)
 	require.NoError(t, err)
 	assert.Equal(t, "xyz", args.Foo)
 }
@@ -71,7 +71,7 @@ func TestCaseSensitive(t *testing.T) {
 		Upper bool `arg:"-V"`
 	}
 
-	err := ParseFrom(&args, split("-v"))
+	err := ParseFrom(split("-v"), &args)
 	require.NoError(t, err)
 	assert.True(t, args.Lower)
 	assert.False(t, args.Upper)
@@ -83,7 +83,7 @@ func TestCaseSensitive2(t *testing.T) {
 		Upper bool `arg:"-V"`
 	}
 
-	err := ParseFrom(&args, split("-V"))
+	err := ParseFrom(split("-V"), &args)
 	require.NoError(t, err)
 	assert.False(t, args.Lower)
 	assert.True(t, args.Upper)
@@ -94,7 +94,7 @@ func TestPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional"`
 	}
-	err := ParseFrom(&args, split("foo"))
+	err := ParseFrom(split("foo"), &args)
 	require.NoError(t, err)
 	assert.Equal(t, "foo", args.Input)
 	assert.Equal(t, "", args.Output)
@@ -105,7 +105,7 @@ func TestRequiredPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional,required"`
 	}
-	err := ParseFrom(&args, split("foo"))
+	err := ParseFrom(split("foo"), &args)
 	assert.Error(t, err)
 }
 
@@ -114,7 +114,7 @@ func TestTooManyPositional(t *testing.T) {
 		Input  string `arg:"positional"`
 		Output string `arg:"positional"`
 	}
-	err := ParseFrom(&args, split("foo bar baz"))
+	err := ParseFrom(split("foo bar baz"), &args)
 	assert.Error(t, err)
 }
 
@@ -123,7 +123,7 @@ func TestMultiple(t *testing.T) {
 		Foo []int
 		Bar []string
 	}
-	err := ParseFrom(&args, split("--foo 1 2 3 --bar x y z"))
+	err := ParseFrom(split("--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)