CLI Enumeration Flags

enumflag/v2 is a Golang package which supplements the Golang CLI flag packages spf13/cobra and spf13/pflag with enumeration flags, including support for enumeration slices. Thanks to Go generics, enumflag/v2 now provides type-safe enumeration flags (and thus requires Go 1.18 or later).

The v2 API is source-compatible with v0 unless you've used the Get() method in the past. However, since the use of Go generics might be a breaking change to downstream projects the semantic major version of enumflag thus went from v0 straight to v2.

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, erm, anything that satisfies constraints.Integers), 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.

Installation

To add enumflag/v2 as a dependency, in your Go module issue:

go get github.com/thediveo/enumflag/v2

How To Use

start with your own enum types,
optional: shell completion,
optional: use existing enum types and non-zero defaults,
optional: CLI flag with default,
optional: CLI flag without a default value,
optional: 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/v2"
)

// 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 satisfies constraints.Integer.
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:

Define your own new enumeration type, such as type FooMode enumflag.Flag.
Define the constants in your enumeration.
Define the mapping of the constants onto enum values (textual representations).
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.
Wire up your flag variable to its flag long and short names, et cetera.

Shell Completion

Dynamic flag completion can be enabled by calling the RegisterCompletion(...) receiver of an enum flag (more precise: flag value) created using enumflag.New(...). enumflag supports dynamic flag completion for both scalar and slice enum flags. Unfortunately, due to the cobra API design it isn't possible for enumflag to offer a fluent API. Instead, creation, adding, and registering have to be carried out as separate instructions.

    // ⑤ Define the CLI flag parameters for your wrapped enum flag.
    ef := enumflag.New(&foomode, "mode", FooModeIds, enumflag.EnumCaseInsensitive)
    rootCmd.PersistentFlags().VarP(
        ef,
        "mode", "m",
        "foos the output; can be 'foo' or 'bar'")
    // ⑥ register completion
    ef.RegisterCompletion(rootCmd, "mode", enumflag.Help[FooMode]{
		Foo: "foos the output",
		Bar: "bars the output",
	})

Please note for shell completion to work, your root command needs to have at least one (explicit) sub command. Otherwise, cobra won't automatically add an additional completion sub command. For more details, please refer to cobra's documentation on Generating shell completions.

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/v2"
)

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 just specifies the CLI flag without 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"

CLI Flag Without Default

In other situations you might not want to have a default value set, because a particular CLI flag is mandatory (using cobra's MarkFlagRequired). Here, cobra's help should not show a (useless) default enum flag setting but only the availabe enum values.

Don't assign the zero value of your enum type to any value, except the "non-existing" default.

// ② Define the enumeration values for FooMode; do not assign the zero value to
// any enum value except for the "no default" default.
const (
    NoDefault FooMode = iota // optional; must be the zero value.
    Foo                      // make sure to not use the zero value.
    Bar
)

Also, don't map the zero value of your enum type.

// ③ Map enumeration values to their textual representations (value
// identifiers).
var FooModeIds = map[FooMode][]string{
    // ...do NOT include/map the "no default" zero value!
    Foo: {"foo"},
    Bar: {"bar"},
}

Finally, simply use enumflag.NewWithoutDefault instead of enumflag.New – that's all.

// ⑤ Define the CLI flag parameters for your wrapped enum flag.
rootCmd.PersistentFlags().VarP(
    enumflag.NewWithoutDefault(&foomode, "mode", FooModeIds, enumflag.EnumCaseInsensitive),
    "mode", "m",
    "foos the output; can be 'foo' or 'bar'")

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/v2"
)

// 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 satisfies constraints.Integer.
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"},
}

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()
}

VSCode Tasks

The included enumflag.code-workspace defines the following tasks:

View Go module documentation task: installs pkgsite, if not done already so, then starts pkgsite and opens VSCode's integrated ("simple") browser to show the go-plugger/v2 documentation.
Build workspace task: builds all, including the shared library test plugin.
Run all tests with coverage task: does what it says on the tin and runs all tests with coverage.

Aux Tasks

pksite service: auxilliary task to run pkgsite as a background service using scripts/pkgsite.sh. The script leverages browser-sync and nodemon to hot reload the Go module documentation on changes; many thanks to @mdaverde's Build your Golang package docs locally for paving the way. scripts/pkgsite.sh adds automatic installation of pkgsite, as well as the browser-sync and nodemon npm packages for the local user.
view pkgsite: auxilliary task to open the VSCode-integrated "simple" browser and pass it the local URL to open in order to show the module documentation rendered by pkgsite. This requires a detour via a task input with ID "pkgsite".

Make Targets

make: lists all targets.
make coverage: runs all tests with coverage and then updates the coverage badge in README.md.
make pkgsite: installs x/pkgsite, as well as the browser-sync and nodemon npm packages first, if not already done so. Then runs the pkgsite and hot reloads it whenever the documentation changes.
make report: installs @gojp/goreportcard if not yet done so and then runs it on the code base.
make test: runs all tests.

Contributing

Please see CONTRIBUTING.md.

thediveo/enumflag