flag

Extensions to github.com/spf13/pflag:

• Typed enum flags — restrict a flag to a fixed set of values, with case-insensitive matching and helpful error messages.
• Positional argument parsing — declare expected positional arguments by name and parse them from the non-flag remainder of os.Args.

Disclaimer: This is a personal project. The views, code, and opinions expressed here are my own and do not represent those of my current or past employers.

Installation

go get bitbucket.org/glennhartmann/flag

Usage

Enum flags

import flag "bitbucket.org/glennhartmann/flag/src/flag"

type Color int

const (
    Red Color = iota
    Green
    Blue
)

func (c Color) String() string {
    return [...]string{"red", "green", "blue"}[c]
}

var colors = []Color{Red, Green, Blue}

color := flag.Enum("color", colors, Red, "output color")
// or with a shorthand:
color := flag.EnumP("color", "c", colors, Red, "output color")

flag.Parse()
fmt.Println(*color) // e.g. "green"

Accepted values are matched case-insensitively against each option's String() representation. An invalid value produces an error listing the allowed options.

Positional arguments

name   := flag.PosString("name",   "", "the name to greet")
target := flag.PosString("target", "", "where to send the greeting")
// or a positional enum:
color  := flag.PosEnum("color", colors, Red, "output color")

flag.Parse() // parses flags, then positional args
fmt.Printf("Hello %s -> %s (in %s)\n", *name, *target, *color)

Positional arguments are matched in registration order. The number of non-flag arguments must match the number of registered positional arguments exactly.

Using a FlagSet directly

All package-level functions have FlagSet method equivalents for use in multi-command programs:

fs := flag.NewFlagSet()
color := flag.FlagSetEnum(fs, "color", colors, Red, "output color")
name  := fs.PosString("name", "", "the name")

fs.Parse()

API overview

Function	Description
Enum[T](name, options, default, usage)	Typed enum flag on CommandLine
EnumP[T](name, short, options, default, usage)	Same, with a shorthand character
PosEnum[T](name, options, default, usage)	Typed positional enum argument
PosString(name, default, usage)	Positional string argument
Pos(val, name, usage)	Positional argument from any pflag.Value
Parse()	Parse flags and positional arguments
ParsePos()	Parse only positional arguments (after pflag.Parse)
ParsePosCustom(args)	Parse positional arguments from a custom slice

FlagSet method variants (FlagSetEnum, FlagSetEnumP, FlagSetPosEnum) and Unsafe variants (erased to fmt.Stringer) are also available.