flagutil

A library to populate flag.FlagSet from various sources.

Features

• Uses standard flag package
• Structured configuration
• Reads values from multiple sources
• Ability to write your own parser

Why

Defining your program parameters and reading their values should be as simple as possible. There is no reason to use some library or even monstrous framework instead of standard flag package.

There is Configuration in Go article, which describes in detail the reasons of creating this library.

Available parsers

Note that it is very easy to implement your own Parser.

At the moment these parsers are already implemented:

• Flag syntax arguments parser
• Posix program arguments syntax parser
• Environment variables parser
• Prompt interactive parser
• File parsers:
  • json
  • yaml
  • toml

Custom help message

It is possible to print custom help message which may include, for example, names of the environment variables used by env parser. See the WithCustomUsage() parse option.

Custom usage currently looks like this:

Usage of test:
  $TEST_FOO, --foo
        bool
        bool flag description (default false)

  $TEST_BAR, --bar
        int
        int flag description (default 42)

Usage

A simple example could be like this:

package main

import (
	"flag"

	"github.com/gobwas/flagutil"
	"github.com/gobwas/flagutil/parse/pargs"
	"github.com/gobwas/flagutil/parse/file/json"
)

func main() {
	flags := flag.NewFlagSet("my-app", flag.ExitOnError)
	
	port := flag.Int(&port,
		"port", "port",
		"port to bind to",
	)

	// This flag will be required by the file.Parser below.
	_ = flags.String(
		"config", "/etc/app/config.json", 
		"path to configuration file",
	)

	flagutil.Parse(flags,
		// Use posix options syntax instead of `flag` – just to illustrate that
		// it is possible.
		flagutil.WithParser(&pargs.Parser{
			Args: os.Args[1:],
		}),	

		// Then lookup flag values among environment.
		flagutil.WithParser(&env.Parser{
			Prefix: "MY_APP_",
		}),

		// Finally lookup for "config" flag value and try to interpret its
		// value as a path to json configuration file.
		flagutil.WithParser(
			&file.Parser{
				Lookup: file.LookupFlag(flags, "config"),
				Syntax: new(json.Syntax),
			},
			// Don't allow to setup "config" flag from file.
			flagutil.WithStashName("config"),
		),
	)

	// Work with received values.
}

Subsets

flagutil provides ability to define so called flag subsets:

package main

import (
	"flag"

	"github.com/gobwas/flagutil"
	"github.com/gobwas/flagutil/parse/pargs"
	"github.com/gobwas/flagutil/parse/file/json"
)

func main() {
	flags := flag.NewFlagSet("my-app", flag.ExitOnError)
	
	port := flag.Int(&port,
		"port", "port",
		"port to bind to",
	)

	// Define parameters for some third-party library.
	var endpoint string
	flagutil.Subset(flags, "database", func(sub *flag.FlagSet) {
		sub.StringVar(&endpoint,
			"endpoint", "localhost",
			"database endpoint to connect to"
		)
	})
	
	flagutil.Parse(flags,
		flagutil.WithParser(&pargs.Parser{
			Args: os.Args[1:],
		}),	
		flagutil.WithParser(
			&file.Parser{
				Lookup: file.PathLookup("/etc/my-app/config.json"),
				Syntax: new(json.Syntax),
			},
		),
	)

	// Work with received values.
}

The configuration file may look as follows:

{
  "port": 4050,
  
  "database": {
    "endpoint": "localhost:5432",
  }
}

And, if you want to override, say, database endpoint, you can execute your program as follows:

$ app --database.endpoint 4055

Allowing name collisions

It's rare, but still possible, when you want to receive single flag value from multiple places in code. To avoid panics with "flag redefined" reason you can (if you really need to) merge two flag values into single by using flagutil.Merge() function:

var (
	s0 string
	s1 string
)
flag.StringVar(&s0,
	"foo", "default",
	"foo flag usage",
)
flagutil.MergeInto(flag.CommandLine, func(safe *flag.FlagSet) {
	safe.StringVar(&s1,
		"foo", "",
		"foo flag another usage", // This usage will be joined with previous.
	)
})

// After parsing, s0 and s1 will be filled with single `-foo` flag value.
// If value is not provided, both s0 and s1 will have its default values (which
// may be _different_).

Conventions and limitations

Any structure from parsed configuration is converted into a pairs of a flat key and a value. Keys are flattened recursively until there is no such flag defined within flag.FlagSet.

Keys flattening happens just as two keys concatenation with . as a > delimiter.

There are three scenarios when the flag was found:

1. If value is a mapping or an object, then its key-value pairs are concatenated with : as a delimiter and are passed to the flag.Value.Set() in appropriate number of calls.

2. If value is an array, then its items are passed to the flag.Value.Set() in appropriate number of calls.

3. In other way, flag.Value.Set() will be called once with value as is.

Note that for any type of values the flag.Value.String() method is never used to access the "real" value – only for defaults when printing help message. To provide "real" value implementations must satisfy flag.Getter interface.

Suppose you have this json configuration:

{
  "foo": {
    "bar": "1",
    "baz": "2"
  }
}

If you define foo.bar flag, you will receive "1" in a single call to its flag.Value.Set() method. No surprise here. But if you define foo flag, then its flag.Value.Set() will be called twice with "bar:1" and "baz:2".

The same thing happens with slices:

{
  "foo": [
    "bar",
    "baz"
  ]
}

Your foo's flag.Value.Set() will be called twice with "bar" and "baz".

This still allows you to use command line arguments to override or declare parameter complex values:

$ app --slice 1 --slice 2 --slice 3 --map foo:bar --map bar:baz

Misc

Creation of this library was greatly inspired by peterburgon/ff – and I wouldn't write flagutil if I didn't met some design disagreement with it.