add support for default values in struct tags

go.mod

@@ -4,3 +4,5 @@ require (
 	github.com/alexflint/go-scalar v1.0.0
 	github.com/stretchr/testify v1.2.2
 )
+
+go 1.13

parse.go

@@ -54,6 +54,7 @@ type spec struct {
 	help       string
 	env        string
 	boolean    bool
+	defaultVal string // default value for this option, only if provided as a struct tag
 }
 
 // command represents a named subcommand, or the top-level command
@@ -250,6 +251,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			spec.help = help
 		}
 
+		defaultVal, hasDefault := field.Tag.Lookup("default")
+		if hasDefault {
+			spec.defaultVal = defaultVal
+		}
+
 		// Look at the tag
 		var isSubcommand bool // tracks whether this field is a subcommand
 		if tag != "" {
@@ -274,6 +280,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 					}
 					spec.short = key[1:]
 				case key == "required":
+					if hasDefault {
+						errs = append(errs, fmt.Sprintf("%s.%s: 'required' cannot be used when a default value is specified",
+							t.Name(), field.Name))
+						return false
+					}
 					spec.required = true
 				case key == "positional":
 					spec.positional = true
@@ -328,6 +339,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 					t.Name(), field.Name, field.Type.String()))
 				return false
 			}
+			if spec.multiple && hasDefault {
+				errs = append(errs, fmt.Sprintf("%s.%s: default values are not supported for slice fields",
+					t.Name(), field.Name))
+				return false
+			}
 		}
 
 		// if this was an embedded field then we already returned true up above
@@ -570,15 +586,26 @@ func (p *Parser) process(args []string) error {
 		return fmt.Errorf("too many positional arguments at '%s'", positionals[0])
 	}
 
-	// finally check that all the required args were provided
+	// fill in defaults and check that all the required args were provided
 	for _, spec := range specs {
-		if spec.required && !wasPresent[spec] {
+		if wasPresent[spec] {
+			continue
+		}
+
 		name := spec.long
 		if !spec.positional {
 			name = "--" + spec.long
 		}
+
+		if spec.required {
 			return fmt.Errorf("%s is required", name)
 		}
+		if spec.defaultVal != "" {
+			err := scalar.ParseValue(p.val(spec.dest), spec.defaultVal)
+			if err != nil {
+				return fmt.Errorf("error processing default value for %s: %v", name, err)
+			}
+		}
 	}
 
 	return nil

parse_test.go

@@ -1057,3 +1057,71 @@ func TestMultipleTerminates(t *testing.T) {
 	assert.Equal(t, []string{"a", "b"}, args.X)
 	assert.Equal(t, "c", args.Y)
 }
+
+func TestDefaultOptionValues(t *testing.T) {
+	var args struct {
+		A int      `default:"123"`
+		B *int     `default:"123"`
+		C string   `default:"abc"`
+		D *string  `default:"abc"`
+		E float64  `default:"1.23"`
+		F *float64 `default:"1.23"`
+		G bool     `default:"true"`
+		H *bool    `default:"true"`
+	}
+
+	err := parse("--c=xyz --e=4.56", &args)
+	require.NoError(t, err)
+
+	assert.Equal(t, 123, args.A)
+	assert.Equal(t, 123, *args.B)
+	assert.Equal(t, "xyz", args.C)
+	assert.Equal(t, "abc", *args.D)
+	assert.Equal(t, 4.56, args.E)
+	assert.Equal(t, 1.23, *args.F)
+	assert.True(t, args.G)
+	assert.True(t, args.G)
+}
+
+func TestDefaultPositionalValues(t *testing.T) {
+	var args struct {
+		A int      `arg:"positional" default:"123"`
+		B *int     `arg:"positional" default:"123"`
+		C string   `arg:"positional" default:"abc"`
+		D *string  `arg:"positional" default:"abc"`
+		E float64  `arg:"positional" default:"1.23"`
+		F *float64 `arg:"positional" default:"1.23"`
+		G bool     `arg:"positional" default:"true"`
+		H *bool    `arg:"positional" default:"true"`
+	}
+
+	err := parse("456 789", &args)
+	require.NoError(t, err)
+
+	assert.Equal(t, 456, args.A)
+	assert.Equal(t, 789, *args.B)
+	assert.Equal(t, "abc", args.C)
+	assert.Equal(t, "abc", *args.D)
+	assert.Equal(t, 1.23, args.E)
+	assert.Equal(t, 1.23, *args.F)
+	assert.True(t, args.G)
+	assert.True(t, args.G)
+}
+
+func TestDefaultValuesNotAllowedWithRequired(t *testing.T) {
+	var args struct {
+		A int `arg:"required" default:"123"` // required not allowed with default!
+	}
+
+	err := parse("", &args)
+	assert.EqualError(t, err, ".A: 'required' cannot be used when a default value is specified")
+}
+
+func TestDefaultValuesNotAllowedWithSlice(t *testing.T) {
+	var args struct {
+		A []int `default:"123"` // required not allowed with default!
+	}
+
+	err := parse("", &args)
+	assert.EqualError(t, err, ".A: default values are not supported for slice fields")
+}