gflag

pflags with generic types.

OK so this is yet another CLI flags library...

We do not reinvent the wheel: this module reuses the great package github.com/spf13/pflag, with an interface built on go generics.

The main idea is to simplify the pflag interface, with less things to remember about flag types.

So this is not a fork or drop-in replacement, but rather an extension of the pflag functionality.

Usage

This package is designed to be used together with github.com/spf13/pflag. All types created by gflag implement the pflag.Value interface.

Use-case: build idiomatic cobra CLIs with a simpler declaration of flags. See how go-cli achieves this.

import (
	"fmt"

	"github.com/fredbi/gflag"
	"github.com/spf13/pflag"
)

var flagVal int

fs := pflag.NewFlagSet("", pflag.ContinueOnError)
intFlag := gflag.NewFlagValue(&flagVal, 1)      // infer flag from underlying type int, with a default value

fs.Var(intFlag, "integer",  "integer value")    // register the flag in pflag flagset

_ = fs.Parse([]string{"--integer", 10})         // parse command line arguments

fmt.Println(intFlag.GetValue())                 // the flag knows the type of the value

With pflag this piece of code would look very much similar.

import (
	"fmt"

	"github.com/spf13/pflag"
)

var flagVal int

fs := pflag.NewFlagSet("", pflag.ContinueOnError)

fs.IntVar(&flagVal, "integer",  "integer value") // register the flag in pflag flagset

_ = fs.Parse([]string{"--integer", 10})         // parse command line arguments

fmt.Println(fs.GetInt("integer"))               // has to know an integer type is expected

You may take a look at more examples, with slices and maps.

What does this bring to the table?

This package provides a unified approach to strongly typed flags, using go generics.

This way, we no longer have to register CLI flags using dozen of type-specific methods, just three:

• NewFlagValue(): for scalar values
• NewFlagSliceValue(): for flags as lists
• NewFlagMapValue(): for flags as map (e.g. key=value pairs)

The flag building logic is consistent for single values, slices and maps of all types. Differences in semantics are handled with a set of Options.

• All primitive types (yes, complex too!) are supported.
• All common types handled by pflag (time.Duration, net.IP, etc..) are also supported.
• I have also added time.Time
• Support any extension built for pflag (arbitrary types with the pflag.Value interface)

Variations in the semantics for a flag with a given underlying type may be fine-tuned with options.

There is an extensions sub-package to contribute some custom extra flag types. It is now populated with a byte-size flag. See the example.

About CLI flags