An illustration of compiling-to-closure

compiler.go

// Suppose you have a simple language with three possible tokens: + - p. The runtime of your language keeps a counter.
// The command + increments the counter, - decrements, and p prints.

type Runtime struct {
  Counter int
}

// An interpreter.
func interprete(r *Runtime, program string) {
  for _, r := range program {
    switch r {
    case '+':
      r.counter++
    case '-':
      r.counter--
    case 'p':
      fmt.Println(r.counter)
    default:
      panic("bad program")
    }
  }
}

// Compile to closure.
func compileProgram(program string) func(*Runtime) {
  var ops []func(*Runtime)
  for _, r := range program {
    ops = append(ops, compileCommand(r))
  }
  return func(r *Runtime) {
    for _, op := range ops {
      op(r)
    }
  }
}

func compileCommand(r rune) func(*Runtime) {
  switch r {
  case '+':
    return func(r *Runtime) { r.counter++ }
  case '-':
    return func(r *Runtime) { r.counter-- }
  case 'p':
    return func(r *Runtime) { fmt.Println(r.counter) }
  default:
    panic("bad program")
  }
}