package main

import (
	"fmt"
	"strings"
)

func foo() string {
	for b, i := (strings.Builder{}), byte('a'); ; i++ {
		b.WriteByte(i)
		if i == 'z' {
			return b.String()
		}
	}
}

func bar(callback func(*strings.Builder)) string {
	for b, i := (strings.Builder{}), byte('a'); ; i++ {
		b.WriteByte(i)
		callback(&b) // <-- difference here
		if i == 'z' {
			return b.String()
		}
	}
}

func main() {
	debugProcess := func(pb *strings.Builder) {
		// do nothing
	}
	fmt.Println("foo:", foo())
	fmt.Println("bar:", bar(debugProcess))
}