1 files changed, 74 insertions, 0 deletions
diff --git a/flagset.go b/flagset.go
new file mode 100644
index 0000000..6118177
--- /dev/null
+++ b/flagset.go
@@ -0,0 +1,74 @@
+// SPDX-FileCopyrightText: 2020 Ethel Morgan
+//
+// SPDX-License-Identifier: MIT
+
+package flag
+
+import (
+	"flag"
+	"fmt"
+	"os"
+)
+
+type (
+	ErrorHandling = flag.ErrorHandling
+
+	ParseFunc func(string) (interface{}, error)
+
+	FlagSet struct {
+		flag.FlagSet
+
+		customFlags []func() error
+	}
+)
+
+const (
+	ContinueOnError = flag.ContinueOnError
+	ExitOnError     = flag.ExitOnError
+	PanicOnError    = flag.PanicOnError
+)
+
+func (f *FlagSet) Parse(arguments []string) error {
+	if err := f.FlagSet.Parse(arguments); err != nil {
+		return err
+	}
+
+	for _, customFlag := range f.customFlags {
+		if err := customFlag(); err != nil {
+			switch f.FlagSet.ErrorHandling() {
+			case flag.ContinueOnError:
+				return err
+			case flag.ExitOnError:
+				fmt.Fprintf(os.Stdout, "%v\n\n", err)
+				f.Usage()
+				os.Exit(2)
+			case flag.PanicOnError:
+				panic(err)
+			}
+		}
+	}
+	return nil
+}
+
+func NewFlagSet(name string, handling ErrorHandling) *FlagSet {
+	return &FlagSet{
+		FlagSet: *flag.NewFlagSet(name, handling),
+	}
+}
+
+func (f *FlagSet) Custom(flagName, defaultValue, description string, parser ParseFunc) *interface{} {
+	rawFlag := f.String(flagName, defaultValue, description)
+
+	var value interface{}
+
+	f.customFlags = append(f.customFlags, func() error {
+		var err error
+		value, err = parser(*rawFlag)
+		if err != nil {
+			return fmt.Errorf("invalid value %q for flag -%s: %w", *rawFlag, flagName, err)
+		}
+		return nil
+	})
+
+	return &value
+}