FSM.tl

fsm.tl

-- fsm.tl

------------------------------------------------------------
-- Common state interface
------------------------------------------------------------

local interface State
   tag: string
end


------------------------------------------------------------
-- Concrete states
--
-- `where` tells Teal how to distinguish each record
-- at runtime when using `is`.
------------------------------------------------------------

local record Idle is State
   where self.tag == "idle"
end

local record Running is State
   where self.tag == "running"

   speed: number
end

local record Jumping is State
   where self.tag == "jumping"

   velocity_y: number
end


------------------------------------------------------------
-- The actual state type
--
-- Not an enum.
-- A state is one of three completely different records.
------------------------------------------------------------

local type PlayerState =
   Idle
   | Running
   | Jumping


------------------------------------------------------------
-- Input to our state machine
------------------------------------------------------------

local record Input
   move: boolean
   jump: boolean
   landed: boolean
end


------------------------------------------------------------
-- Constructors
------------------------------------------------------------

local function idle(): Idle
   return {
      tag = "idle",
   }
end

local function running(speed: number): Running
   return {
      tag = "running",
      speed = speed,
   }
end

local function jumping(velocity_y: number): Jumping
   return {
      tag = "jumping",
      velocity_y = velocity_y,
   }
end


------------------------------------------------------------
-- Transition function
------------------------------------------------------------

local function step(
   state: PlayerState,
   input: Input
): PlayerState

   if state is Idle then

      -- state is statically known to be Idle here

      if input.jump then
         return jumping(12)
      end

      if input.move then
         return running(5)
      end

      return state


   elseif state is Running then

      -- state is statically Running here.
      -- Therefore Running-only data is available:
      print("running at", state.speed)

      if input.jump then
         return jumping(12)
      end

      if not input.move then
         return idle()
      end

      return state


   elseif state is Jumping then

      -- state is statically Jumping here.
      print("vertical velocity:", state.velocity_y)

      if input.landed then
         return idle()
      end

      return state
   end

   return state
end


------------------------------------------------------------
-- Display helper
------------------------------------------------------------

local function show(state: PlayerState)
   if state is Idle then
      print("IDLE")

   elseif state is Running then
      print("RUNNING, speed =", state.speed)

   elseif state is Jumping then
      print("JUMPING, vy =", state.velocity_y)
   end
end


------------------------------------------------------------
-- Run it
------------------------------------------------------------

local state: PlayerState = idle()

show(state)
-- IDLE


state = step(state, {
   move = true,
   jump = false,
   landed = false,
})

show(state)
-- RUNNING, speed = 5


state = step(state, {
   move = true,
   jump = true,
   landed = false,
})

show(state)
-- JUMPING, vy = 12


state = step(state, {
   move = false,
   jump = false,
   landed = true,
})

show(state)
-- IDLE