readme.md

These are all bad examples as you would use it on the I/O and other things which would change through the stack and as a result fairly contrived.

main1.go

package main

import (
  "fmt"
)

// Library

context (
  logprefix string = ""
)

func SetPrefix(s string) {
  logprefix = s
}

func Log(s string) {
  fmt.Printf("%s: %s", logprefix, s)
}

// Client

func c(){}

func b() {
  SetPrefix("[B]")
  Log("2")
  c()
  Log("2.1")
}

func a() {
  SetPrefix("[A]")
  Log("1")
  b()
  Log("1.1")
}

func main() {
  a()
}

main2.go

package main

import (
  "fmt"
)

// Library

var (
  logprefix string = ""
)

func SetPrefix(s string) {
  logprefix = s
}

func GetPrefix() string {
  return logprefix
}

func Log(s string) {
  fmt.Printf("%s: %s", logprefix, s)
}

// Client

func c(){}

func b() {
  p := GetPrefix()
  SetPrefix("[B]")
  defer func() {
    SetPrefix(p)
  }
  Log("2")
  c()
  Log("2.1")
}

func a() {
  p := GetPrefix()
  SetPrefix("[A]")
  defer func() {
    SetPrefix(p)
  }
  Log("1")
  b()
  Log("1.1")
}

func main() {
  a()
}

main3.go

package main

import (
  "fmt"
)

// Library

var (
  logprefix string = ""
)

type PreviousValue string

func (p PreviousValue) Close() error {
  logprefix = string(p)
  return nil
}

func SetPrefix(s string) PreviousValue {
  pv, logprefix = PreviousValue(logprefix), s
  return pv
}

func Log(s string) {
  fmt.Printf("%s: %s", logprefix, s)
}

// Client

func c(){}

func b() {
  p := SetPrefix("[B]")
  defer p.Close()
  defer func() {
    SetPrefix(p)
  }
  Log("2")
  c()
  Log("2.1")
}

func a() {
  p := SetPrefix("[A]")
  defer p.Close()
  Log("1")
  b()
  Log("1.1")
}

func main() {
  a()
}

main4.go

package main

import (
  "fmt"
)

// Library

func Log(s string) {
  fmt.Printf("%s", s)
}

// Client

func c(){}

func b(prefix string) {
  prefix = prefix + "[A]"
  Log(prefix + ": 2")
  c()
  Log(prefix + ": 2.1")
}

func a(prefix string) {
  np := prefix + "[B]"
  Log(np + ": 1")
  b(prefix)
  Log(np + ": 1.1")
}

func main() {
  a("")
}

main5.go

package main

import (
  "fmt"
)

// Library

type Logger struct {
  Prefix string 
}

func (l *Logger) Log(s string) {
  fmt.Printf("%s: %s", l.Prefix, s)
}

// Client

func c(){}

func b(l *Logger) {
  l.Prefix = "[B]"
  l.Log(prefix + ": 2")
  c()
  l.Log(prefix + ": 2.1")
}

func a(l *Logger) {
  l.Prefix = "[A]"
  Log(np + ": 1")
  b(l)
  l.Prefix = "[A]"
  Log(np + ": 1.1")
}

func main() {
  l := &Logger{
    Prefix: "",
  }
  a(l)
}