1 files changed, 84 insertions, 0 deletions
diff --git a/flag.go b/flag.go
new file mode 100644
index 0000000..bed32eb
--- /dev/null
+++ b/flag.go
@@ -0,0 +1,84 @@
+// SPDX-FileCopyrightText: © 2022 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package core
+
+import (
+	"flag"
+	"fmt"
+	"time"
+)
+
+func FlagVar[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T]) *T {
+	v := &flagValue[T]{Parse: parse, Value: new(T)}
+	fs.Var(v, name, usage)
+	return v.Value
+}
+
+func FlagVarPtr[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T], val *T) {
+	fs.Var(&flagValue[T]{Parse: parse, Value: val}, name, usage)
+}
+
+func FlagVarSlice[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T]) *[]T {
+	v := &flagValueSlice[T]{Parse: parse, Values: new([]T)}
+	fs.Var(v, name, usage)
+	return v.Values
+}
+
+func FlagVarSlicePtr[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T], vals *[]T) {
+	fs.Var(&flagValueSlice[T]{Parse: parse, Values: vals}, name, usage)
+}
+
+// ParseString returns the string passed with no error set.
+func ParseString(s string) (string, error) {
+	return s, nil
+}
+
+// ParseTime parses a string according to the time.RFC3339 format.
+func ParseTime(s string) (time.Time, error) {
+	return time.Parse(time.RFC3339, s)
+}
+
+// ParseFunc describes functions that will parse a string and return a
+// value or an error.
+type ParseFunc[T any] func(string) (T, error)
+
+type flagValue[T any] struct {
+	Parse ParseFunc[T]
+	Value *T
+}
+
+var _ flag.Value = &flagValue[any]{}
+
+func (f *flagValue[T]) Set(s string) error {
+	val, err := f.Parse(s)
+	if err != nil {
+		return err
+	}
+	*f.Value = val
+	return nil
+}
+
+func (f *flagValue[T]) String() string {
+	return fmt.Sprintf("%v", f.Value)
+}
+
+type flagValueSlice[T any] struct {
+	Parse  ParseFunc[T]
+	Values *[]T
+}
+
+var _ flag.Value = &flagValueSlice[any]{}
+
+func (f *flagValueSlice[T]) Set(s string) error {
+	val, err := f.Parse(s)
+	if err != nil {
+		return err
+	}
+	*f.Values = append(*f.Values, val)
+	return nil
+}
+
+func (f *flagValueSlice[T]) String() string {
+	return fmt.Sprintf("%v", f.Values)
+}