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flagutil

★ 37 Open GitHub ↗

Easy flags population.

// repository documentation

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# flagutil

[![GoDoc][godoc-image]][godoc-url]
[![CI][ci-badge]][ci-url]

> A library to populate [`flag.FlagSet`][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][flag] package.

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

# Available parsers

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

At the moment these parsers are already implemented:
- [Flag][flag-syntax] syntax arguments parser
- [Posix][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:

```go
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:

```go
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:

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

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

```bash
$ 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:

```go
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:

```json
{
  "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:

```json
{
  "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:

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

# Misc

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

[parser]:      https://godoc.org/github.com/gobwas/flagutil#Parser
[flag]:        https://golang.org/pkg/flag
[flagSet]:     https://golang.org/pkg/flag#FlagSet
[flag-syntax]: https://golang.org/pkg/flag/#hdr-Command_line_flag_syntax
[article]:     https://gbws.io/articles/configuration-in-go
[godoc-image]: https://godoc.org/github.com/gobwas/flagutil?status.svg
[godoc-url]:   https://godoc.org/github.com/gobwas/flagutil
[posix]:       https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html
[ff]:          https://github.com/peterbourgon/ff
[ci-badge]:    https://github.com/gobwas/flagutil/workflows/CI/badge.svg
[ci-url]:      https://github.com/gobwas/flagutil/actions?query=workflow%3ACI

flagutil

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