command: implement builder and parser

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/flag.go

@@ -0,0 +1,49 @@
+package command
+
+import (
+	"errors"
+	"flag"
+	"strings"
+)
+
+// FlagError wraps errors returned by [flag].
+type FlagError struct{ error }
+
+func (e FlagError) Success() bool { return errors.Is(e.error, flag.ErrHelp) }
+func (e FlagError) Is(target error) bool {
+	return (e.error == nil && target == nil) ||
+		((e.error != nil && target != nil) && e.error.Error() == target.Error())
+}
+
+func (n *node) Flag(p any, name string, value FlagDefiner, usage string) Node {
+	value.Define(&n.suffix, n.set, p, name, usage)
+	return n
+}
+
+// StringFlag is the default value of a string flag.
+type StringFlag string
+
+func (v StringFlag) Define(b *strings.Builder, set *flag.FlagSet, p any, name, usage string) {
+	set.StringVar(p.(*string), name, string(v), usage)
+	b.WriteString(" [" + prettyFlag(name) + " <value>]")
+}
+
+// BoolFlag is the default value of a bool flag.
+type BoolFlag bool
+
+func (v BoolFlag) Define(b *strings.Builder, set *flag.FlagSet, p any, name, usage string) {
+	set.BoolVar(p.(*bool), name, bool(v), usage)
+	b.WriteString(" [" + prettyFlag(name) + "]")
+}
+
+// this has no effect on parse outcome
+func prettyFlag(name string) string {
+	switch len(name) {
+	case 0:
+		panic("zero length flag name")
+	case 1:
+		return "-" + name
+	default:
+		return "--" + name
+	}
+}