Golang: recursive types v2

Mike

2 min read

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1 day ago

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In the previous post, I wrote about recursive types: maps, slices, structs, and channels. Structs, maps, and slices show up in real data.
A recursive channel is mostly an interview topic.

Let’s continue.

Did you know this signature is valid?

type Func func() Func

A function type that refers to itself.

How do you use that in real code? Is it useful?

Yes, if you don’t like if-else spaghetti.

In the previous post, I mentioned PBEM (Play-by-Email) games
and noticed that each step had no World. Let’s fix that.

First, the world and a step over it:

type World struct {
  TurnID     int
  HasWinner  bool
  NextPlayer string
}

type StepFn func(*World) StepFn

Then, a few steps:

// waiting for the email move
func waitForEmailStep(w *World) StepFn {
  fmt.Printf("[Turn %d]: Waiting for %s's email move...\n", w.TurnID, w.NextPlayer)
  return processMoveStep
}

func processMoveStep(w *World) StepFn {
  fmt.Println("[Engine]: Calculating ... etc.")

  if w.TurnID >= 3 {
    w.HasWinner = true
    return endGameStep
  }

  w.TurnID++
  if w.NextPlayer == "Player #1" {
    w.NextPlayer = "Player #2"
  } else {
    w.NextPlayer = "Player #1"
  }
  return sendNotificationsStep
}

// jump back to waiting
func sendNotificationsStep(w *World) StepFn {
  fmt.Printf("[Email]: Sending new save game file to %s ...\n", w.NextPlayer)
  return waitForEmailStep
}

// base case: nil breaks the loop
func endGameStep(w *World) StepFn {
  fmt.Println("[Game Over]: FINAL RESULTS.")
  return nil
}

Nothing magic here. These are just the actions the game needs in order to progress.

And the driver:

func main() {
  world := &World{TurnID: 1, NextPlayer: "Player #1"}

  state := waitForEmailStep

  for state != nil {
    state = state(world)
  }
}

The useful part is isolation, and how small the control flow stays.
There is no package-level state. The only shared state is the World itself.
You can easily recombine the functions or insert a new step. New game release!

Full code is here.

This is a more or less real example.
For the same idea in the standard library,
see text/template/parse/lex.go where Rob Pike uses type stateFn func(*lexer) stateFn to build a clean state machine for the template lexer.

Happy coding!