CLI Enumeration Flags

enumflag is a Golang package which supplements the Golang CLI flag packages spf13/cobra and spf13/pflag with enumeration flags, including support for enumeration slices.

For instance, users can specify enum flags as --mode=foo or --mode=bar, where foo and bar are valid enumeration values. Other values which are not part of the set of allowed enumeration values cannot be set and raise CLI flag errors. In case of an enumeration slice flag users can specify multiple enumeration values either with a single flag --mode=foo,bar or multiple flag calls, such as --mode=foo --mode=bar.

Application programmers then simply deal with enumeration values in form of uints (or ints), liberated from parsing strings and validating enumeration flags.

Alternatives

In case you are just interested in string-based one-of-a-set flags, then the following packages offer you a minimalist approach:

• hashicorp/packer/helper/enumflag really is a reduced-to-the-max version without any whistles and bells.

• creachadair/goflags/enumflag has a similar, but slightly more elaborate API with additional "indices" for enumeration values.

But if you instead want to handle one-of-a-set flags as properly typed enumerations instead of strings, or if you need (multiple-of-a-set) slice support, then please read on.

How To Use

• start with your own enum types,
• use existing enum types and non-zero defaults,
• CLI flag with default,
• slice of enums.

Start With Your Own Enum Types

Without further ado, here's how to define and use enum flags in your own applications...

import (
    "fmt"

    "github.com/spf13/cobra"
    "github.com/thediveo/enumflag"
)

// ① Define your new enum flag type. It can be derived from enumflag.Flag, but
// it doesn't need to be as long as it is compatible with enumflag.Flag, so
// either an int or uint.
type FooMode enumflag.Flag

// ② Define the enumeration values for FooMode.
const (
    Foo FooMode = iota
    Bar
)

// ③ Map enumeration values to their textual representations (value
// identifiers).
var FooModeIds = map[FooMode][]string{
    Foo: {"foo"},
    Bar: {"bar"},
}

// ④ Now use the FooMode enum flag. If you want a non-zero default, then simply
// set it here, such as in "foomode = Bar".
var foomode FooMode

func main() {
    rootCmd := &cobra.Command{
        Run: func(cmd *cobra.Command, _ []string) {
            fmt.Printf("mode is: %d=%q\n",
                foomode,
                cmd.PersistentFlags().Lookup("mode").Value.String())
        },
    }
    // ⑤ Define the CLI flag parameters for your wrapped enum flag.
    rootCmd.PersistentFlags().VarP(
        enumflag.New(&foomode, "mode", FooModeIds, enumflag.EnumCaseInsensitive),
        "mode", "m",
        "foos the output; can be 'foo' or 'bar'")

    rootCmd.SetArgs([]string{"--mode", "bAr"})
    _ = rootCmd.Execute()
}

The boilerplate pattern is always the same:

1. Define your own new enumeration type, such as type FooMode enumflag.Flag.
2. Define the constants in your enumeration.
3. Define the mapping of the constants onto enum values (textual representations).
4. Somewhere, declare a flag variable of your enum flag type.
   • If you want to use a non-zero default enum value, just go ahead and set it: var foomode = Bar. It will be used correctly.
5. Wire up your flag variable to its flag long and short names, et cetera.

Use Existing Enum Types

A typical example might be your application using a 3rd party logging package and you want to offer a -v log level CLI flag. Here, we use the existing 3rd party enum values and set a non-zero default for our logging CLI flag.

Considering the boiler plate shown above, we can now leave out steps ① and ②, because these definitions come from a 3rd party package. We only need to supply the textual enum names as ③.

import (
    "fmt"
    "os"

    log "github.com/sirupsen/logrus"
    "github.com/spf13/cobra"
    "github.com/thediveo/enumflag"
)

func main() {
    // ①+② skip "define your own enum flag type" and enumeration values, as we
    // already have a 3rd party one.

    // ③ Map 3rd party enumeration values to their textual representations
    var LoglevelIds = map[log.Level][]string{
        log.TraceLevel: {"trace"},
        log.DebugLevel: {"debug"},
        log.InfoLevel:  {"info"},
        log.WarnLevel:  {"warning", "warn"},
        log.ErrorLevel: {"error"},
        log.FatalLevel: {"fatal"},
        log.PanicLevel: {"panic"},
    }

    // ④ Define your enum flag value and set the your logging default value.
    var loglevel log.Level = log.WarnLevel

    rootCmd := &cobra.Command{
        Run: func(cmd *cobra.Command, _ []string) {
            fmt.Printf("logging level is: %d=%q\n",
                loglevel,
                cmd.PersistentFlags().Lookup("log").Value.String())
        },
    }

    // ⑤ Define the CLI flag parameters for your wrapped enum flag.
    rootCmd.PersistentFlags().Var(
        enumflag.New(&loglevel, "log", LoglevelIds, enumflag.EnumCaseInsensitive),
        "log",
        "sets logging level; can be 'trace', 'debug', 'info', 'warn', 'error', 'fatal', 'panic'")

    // Defaults to what we set above: warn level.
    _ = rootCmd.Execute()

    // User specifies a specific level, such as log level. 
    rootCmd.SetArgs([]string{"--log", "debug"})
    _ = rootCmd.Execute()
}

CLI Flag With Default

Sometimes you might want a CLI enum flag to have a default value when the user specifies the CLI flag, but not its value. A good example is the --color flag of the ls command:

• if just specified as --color without a value, it will default to the value of auto;
• otherwise, as specific value can be given, such as
  • --color=always,
  • --color=never,
  • or even --color=auto.

In such situations, use spf13/pflags's NoOptDefVal to set the flag's default value as text, if the flag is on the command line without any options.

The gist here is as follows, please see also colormode.go from my lxkns Linux namespaces discovery project:

rootCmd.PersistentFlags().VarP(
    enumflag.New(&colorize, "color", colorModeIds, enumflag.EnumCaseSensitive),
    "color", "c",
    "colorize the output; can be 'always' (default if omitted), 'auto',\n"+
        "or 'never'")
rootCmd.PersistentFlags().Lookup("color").NoOptDefVal = "always"

Slice of Enums

For a slice of enumerations, simply declare your variable to be a slice of your enumeration type and then use enumflag.NewSlice(...) instead of enumflag.New(...).

import (
    "fmt"

    "github.com/spf13/cobra"
    "github.com/thediveo/enumflag"
)

// ① Define your new enum flag type. It can be derived from enumflag.Flag, but
// it doesn't need to be as long as it is compatible with enumflag.Flag, so
// either an int or uint.
type MooMode enumflag.Flag

// ② Define the enumeration values for FooMode.
const (
    Moo MooMode = (iota + 1) * 111
    Møø
    Mimimi
)

// ③ Map enumeration values to their textual representations (value
// identifiers).
var MooModeIds = map[MooMode][]string{
    Moo:    {"moo"},
    Møø:    {"møø"},
    Mimimi: {"mimimi"},
}

// User-defined enum flag types should be derived from "enumflag.Flag"; however
// this is not strictly necessary as long as they can be converted into the
// "enumflag.Flag" type. Actually, "enumflag.Flag" is just a fancy name for an
// "uint". In order to use such user-defined enum flags as flag slices, simply
// wrap them using enumflag.NewSlice.
func Example_slice() {
    // ④ Define your enum slice flag value.
    var moomode []MooMode
    rootCmd := &cobra.Command{
        Run: func(cmd *cobra.Command, _ []string) {
            fmt.Printf("mode is: %d=%q\n",
                moomode,
                cmd.PersistentFlags().Lookup("mode").Value.String())
        },
    }
    // ⑤ Define the CLI flag parameters for your wrapped enumm slice flag.
    rootCmd.PersistentFlags().VarP(
        enumflag.NewSlice(&moomode, "mode", MooModeIds, enumflag.EnumCaseInsensitive),
        "mode", "m",
        "can be any combination of 'moo', 'møø', 'mimimi'")

    rootCmd.SetArgs([]string{"--mode", "Moo,møø"})
    _ = rootCmd.Execute()
}

Copyright and License

lxkns is Copyright 2020 Harald Albrecht, and licensed under the Apache License, Version 2.0.