refactor validation

parse.go

@@ -24,7 +24,6 @@ type spec struct {
 	separate   bool
 	help       string
 	env        string
-	wasPresent bool
 	boolean    bool
 }
 
@@ -80,7 +79,7 @@ type Config struct {
 
 // Parser represents a set of command line options with destination values
 type Parser struct {
-	spec        []*spec
+	specs       []*spec
 	config      Config
 	version     string
 	description string
@@ -214,7 +213,7 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 					}
 				}
 			}
-			p.spec = append(p.spec, &spec)
+			p.specs = append(p.specs, &spec)
 
 			// if this was an embedded field then we already returned true up above
 			return false
@@ -250,21 +249,18 @@ func (p *Parser) Parse(args []string) error {
 	}
 
 	// Process all command line arguments
-	err := process(p.spec, args)
-	if err != nil {
-		return err
-	}
-
-	// Validate
-	return validate(p.spec)
+	return p.process(args)
 }
 
 // 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 {
+func (p *Parser) process(args []string) error {
+	// track the options we have seen
+	wasPresent := make(map[*spec]bool)
+
 	// construct a map from --option to spec
 	optionMap := make(map[string]*spec)
-	for _, spec := range specs {
+	for _, spec := range p.specs {
 		if spec.positional {
 			continue
 		}
@@ -274,8 +270,19 @@ func process(specs []*spec, args []string) error {
 		if spec.short != "" {
 			optionMap[spec.short] = spec
 		}
-		if spec.env != "" {
-			if value, found := os.LookupEnv(spec.env); found {
+	}
+
+	// deal with environment vars
+	for _, spec := range p.specs {
+		if spec.env == "" {
+			continue
+		}
+
+		value, found := os.LookupEnv(spec.env)
+		if !found {
+			continue
+		}
+
 		if spec.multiple {
 			// expect a CSV string in an environment
 			// variable in the case of multiple values
@@ -299,9 +306,7 @@ func process(specs []*spec, args []string) error {
 				return fmt.Errorf("error processing environment variable %s: %v", spec.env, err)
 			}
 		}
-				spec.wasPresent = true
-			}
-		}
+		wasPresent[spec] = true
 	}
 
 	// process each string from the command line
@@ -334,7 +339,7 @@ func process(specs []*spec, args []string) error {
 		if !ok {
 			return fmt.Errorf("unknown argument %s", arg)
 		}
-		spec.wasPresent = true
+		wasPresent[spec] = true
 
 		// deal with the case of multiple values
 		if spec.multiple {
@@ -382,20 +387,21 @@ func process(specs []*spec, args []string) error {
 	}
 
 	// process positionals
-	for _, spec := range specs {
+	for _, spec := range p.specs {
 		if !spec.positional {
 			continue
 		}
-		if spec.required && len(positionals) == 0 {
-			return fmt.Errorf("%s is required", spec.long)
+		if len(positionals) == 0 {
+			break
 		}
+		wasPresent[spec] = true
 		if spec.multiple {
 			err := setSlice(spec.dest, positionals, true)
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", spec.long, err)
 			}
 			positionals = nil
-		} else if len(positionals) > 0 {
+		} else {
 			err := scalar.ParseValue(spec.dest, positionals[0])
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", spec.long, err)
@@ -406,6 +412,18 @@ func process(specs []*spec, args []string) error {
 	if len(positionals) > 0 {
 		return fmt.Errorf("too many positional arguments at '%s'", positionals[0])
 	}
+
+	// finally check that all the required args were provided
+	for _, spec := range p.specs {
+		if spec.required && !wasPresent[spec] {
+			name := spec.long
+			if !spec.positional {
+				name = "--" + spec.long
+			}
+			return fmt.Errorf("%s is required", name)
+		}
+	}
+
 	return nil
 }
 
@@ -427,16 +445,6 @@ func isFlag(s string) bool {
 	return strings.HasPrefix(s, "-") && strings.TrimLeft(s, "-") != ""
 }
 
-// validate an argument spec after arguments have been parse
-func validate(spec []*spec) error {
-	for _, arg := range spec {
-		if !arg.positional && arg.required && !arg.wasPresent {
-			return fmt.Errorf("--%s is required", arg.long)
-		}
-	}
-	return nil
-}
-
 // parse a value as the appropriate type and store it in the struct
 func setSlice(dest reflect.Value, values []string, trunc bool) error {
 	if !dest.CanSet() {

parse_test.go

@@ -969,3 +969,18 @@ func TestSpacesAllowedInTags(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, []string{"one", "two", "three", "four"}, args.Foo)
 }
+
+func TestReuseParser(t *testing.T) {
+	var args struct {
+		Foo string `arg:"required"`
+	}
+
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"--foo=abc"})
+	assert.Equal(t, args.Foo, "abc")
+
+	err = p.Parse([]string{})
+	assert.Error(t, err)
+}

usage.go

@@ -1,12 +1,12 @@
 package arg
 
 import (
+	"encoding"
 	"fmt"
 	"io"
 	"os"
 	"reflect"
 	"strings"
-	"encoding"
 )
 
 // the width of the left column
@@ -22,7 +22,7 @@ func (p *Parser) Fail(msg string) {
 // WriteUsage writes usage information to the given writer
 func (p *Parser) WriteUsage(w io.Writer) {
 	var positionals, options []*spec
-	for _, spec := range p.spec {
+	for _, spec := range p.specs {
 		if spec.positional {
 			positionals = append(positionals, spec)
 		} else {
@@ -72,7 +72,7 @@ func (p *Parser) WriteUsage(w io.Writer) {
 // WriteHelp writes the usage string followed by the full help string for each option
 func (p *Parser) WriteHelp(w io.Writer) {
 	var positionals, options []*spec
-	for _, spec := range p.spec {
+	for _, spec := range p.specs {
 		if spec.positional {
 			positionals = append(positionals, spec)
 		} else {