How to react to keypress events in Go?

I have a REPL app in Go that should react to keyboard press events (distinct action for each key pressed key) but ReadString expects for <kbd>return<kbd> key to be pressed before reading os.Stdin:

import (
	"bufio"
	"os"
)

for {
    reader := bufio.NewReader(os.Stdin)
    key, _ := reader.ReadString('\n')
    deferKey(key)
}

How can I react to key press events in Go?

Game engines commonly implement this kind of functionality. They are usually also pretty much platform agnostic (usually at least Windows, Linux, Mac OS X). Try for instance Azul3D's keyboard library.

The logic is off the top of my head something like

watcher := keyboard.NewWatcher()
// Query for the map containing information about all keys
status := watcher.States()
left := status[keyboard.ArrowLeft]
if left == keyboard.Down {
    // The arrow to left is being held down
    // Do something!
}

Getting a list of what keys are currently being pressed down is a matter of iterating the map through and listing the keys where value was Down.

This answer for a similar question points to few options depending on which platform you need to implement this in.

I've personally used https://github.com/MarinX/keylogger.
It's well written and easy to understand. At the time, I had to write my own version of this library to listen to multiple keyboards, It was easy to adapt this code for that.
Note that this library is written for Linux only.

Example code:
<!-- language: go -->

import (
	"github.com/MarinX/keylogger"
	"github.com/sirupsen/logrus"
)

func main() {

	// find keyboard device, does not require a root permission
	keyboard := keylogger.FindKeyboardDevice()

	logrus.Println("Found a keyboard at", keyboard)
	// init keylogger with keyboard
	k, err := keylogger.New(keyboard)
	if err != nil {
		logrus.Error(err)
		return
	}
	defer k.Close()

	events := k.Read()

	// range of events
	for e := range events {
		switch e.Type {
		// EvKey is used to describe state changes of keyboards, buttons, or other key-like devices.
		// check the input_event.go for more events
		case keylogger.EvKey:

			// if the state of key is pressed
			if e.KeyPress() {
				logrus.Println("[event] press key ", e.KeyString())
			}

			// if the state of key is released
			if e.KeyRelease() {
				logrus.Println("[event] release key ", e.KeyString())
			}

			break
		}
	}
}