Implementing auto autocomplete for CLI application - go

Question

I am looking into writing a CLI application in Go.
One of the requirements is auto complete. Not of the command itself but of possible options.
Imagine I want to add a new entry using the CLI. Each entry can have a category.
The categories are available in a slice. What I want to do now is to enable the user to tab through the available categories when typing in add.
I am aware of libraries like https://github.com/chzyer/readline and https://github.com/spf13/cobra but could not find if or how they support this.

Answer 1

Thank you #ain and #JimB for pointing me in the right direction.
Based on the example provided at https://github.com/chzyer/readline/tree/master/example/readline-demo I was able to achieve the desired functionality.
The following code has to main commands newEntry and newCategory. If the user types newEntry and than presses TAB he can choose from the available categories. The newCategory command allow to add a new custom category which is immediately available the next time newEntry is executed.
package main
import (
"io"
"log"
"strconv"
"strings"
"github.com/chzyer/readline"
)
// completer defines which commands the user can use
var completer = readline.NewPrefixCompleter()
// categories holding the initial default categories. The user can add categories.
var categories = []string{"Category A", "Category B", "Category C"}
var l *readline.Instance
func main() {
// Initialize config
config := readline.Config{
Prompt: "\033[31m»\033[0m ",
HistoryFile: "/tmp/readline.tmp",
AutoComplete: completer,
InterruptPrompt: "^C",
EOFPrompt: "exit",
HistorySearchFold: true,
}
var err error
// Create instance
l, err = readline.NewEx(&config)
if err != nil {
panic(err)
}
defer l.Close()
// Initial initialization of the completer
updateCompleter(categories)
log.SetOutput(l.Stderr())
// This loop watches for user input and process it
for {
line, err := l.Readline()
if err == readline.ErrInterrupt {
if len(line) == 0 {
break
} else {
continue
}
} else if err == io.EOF {
break
}
line = strings.TrimSpace(line)
// Checking which command the user typed
switch {
// Add new category
case strings.HasPrefix(line, "newCategory"):
// Remove the "newCategory " prefix (including space)
if len(line) <= 12 {
log.Println("newCategory <NameOfCategory>")
break
}
// Append everything that comes behind the command as the name of the new category
categories = append(categories, line[12:])
// Update the completer to make the new category available in the cmd
updateCompleter(categories)
// Program is closed when user types "exit"
case line == "exit":
goto exit
// Log all commands we don't know
default:
log.Println("Unknown command:", strconv.Quote(line))
}
}
exit:
}
// updateCompleter is updates the completer allowing to add new command during runtime. The completer is recreated
// and the configuration of the instance update.
func updateCompleter(categories []string) {
var items []readline.PrefixCompleterInterface
for _, category := range categories {
items = append(items, readline.PcItem(category))
}
completer = readline.NewPrefixCompleter(
readline.PcItem("newEntry",
items...,
),
readline.PcItem("newCategory"),
)
l.Config.AutoComplete = completer
}