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Barebones subcommand handling

// repository documentation

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flagg
-----

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```
go get lukechampine.com/flagg
```

`flagg` is a tiny package that makes it easier to build CLI apps that use
subcommands. I built it because the stdlib `flag` package is too low-level,
but popular alternatives like `spf13/cobra` are full-fledged frameworks with
too many bells and whistles for my liking. `flagg` is 67 lines of code and
imports only stdlib packages.

`flagg` is designed around the stdlib `*flag.FlagSet` type. You represent each
subcommand in your app with a `*flag.FlagSet`, and arrange them into a hierarchy
with `flagg.Tree`. Then just call `flagg.Parse` to parse the subcommand selected
by `os.Args`.

## Example

```go
// commands are just *flag.FlagSets
var rootCmd *flag.FlagSet = flagg.Root
rootCmd.Usage = flagg.SimpleUsage(rootCmd, "An example program")
verbose := rootCmd.Bool("v", false, "display verbose output")

// flagg.New constructs a *flag.FlagSet with the given name and usage
// description
fooCmd := flagg.New("foo", "The foo subcommand")
bars := fooCmd.Int("bars", 0, "number of bars to foo")
quuxCmd := flagg.New("quux", "The quux subcommand")

// construct the command hierarchy
tree := flagg.Tree{
	Cmd: rootCmd,
	Sub: []flagg.Tree{
		{Cmd: fooCmd},
		{Cmd: quuxCmd},
	},
}

// Parse the command hierarchy. cmd is the selected command, e.g. if
// os.Args = []string{"./example", "quux"}, then cmd == quuxCmd.
cmd := flagg.Parse(tree)

// again, cmd is just a *flag.FlagSet, so Args() returns the arguments of the
// selected command.
args := cmd.Args()

// use a switch to identify the cmd that was selected
switch cmd {
case fooCmd:
	fmt.Printf("fooing %v bars\n", *bars)
case quuxCmd:
	fmt.Println("quux!", args)
}
```

flagg

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