"Declared but not used" variable in for's condition expression

My intuitive approach:
https://play.golang.org/p/6xzg7TK1IH

and it doesn't work.

Can you share some alternative ways?

You are declaring a variable (moreline) which you don't use it. You have two options here: either to replace the moreline with underscore, which means you can omit the return value.

for moreline {
        line, _, err := bio.ReadLine()
        if err != nil {
                log.Fatal(err)
        }
        fmt.Println(line)
}

But a better option will be to use the ReadScanner, ReadBytes('\n') or ReadString('\n').

Checking the bufio.go file this is what you get as description for the ReadLine method:

> ReadLine is a low-level line-reading primitive. Most callers should
> use ReadBytes('\n') or ReadString('\n') instead or use a Scanner.

Calling UnreadByte after ReadLine will always unread the last byte read (possibly a character belonging to the line end) even if that byte is not part of the line returned by ReadLine. ReadLine either returns a non-nil line or it returns an error, never both.

So this would be a better option:

scanner := bufio.NewScanner(bio)

for scanner.Scan() {
	line := scanner.Text()
	fmt.Printf("%v\n", line)
}

You use := which discards the previous contents of the variables. You don't use morelines in that scope, thus the error message.

If you declare your variables beforehand and don't use :=, it works fine. The function ReadLine() might not do what you think it should do.

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	bio := bufio.NewReader(os.Stdin)
	var line []byte
	var err error
	moreline := true
	for moreline {
		line, moreline, err = bio.ReadLine()
		if err != nil {
			log.Fatal(err)
		}
		fmt.Println(line)
	}
}