State I

Many variations are possible in creating a function to represent an object with state, e.g.

Obj :List Cmd -> List Ans

where the commands (Cmd) are to set a state, or to query (get) the current state, and the answers (Ans) are ok, (st,State) or err.

The initial state is hard-wired here, but could be an initial parameter of Obj.

If we concentrate on the state transformations, we have

and indeed the above is sufficient if the output "file", or answer, is made a part of the state, together with the values of the true state variables. (For an alternative, see [state 2].)

let Obj = lambda Cmds.  { List Cmd -> List Ans }

  let rec
  process = lambda state. lambda Cmds.
    if null Cmds then nil else
    let rec Cmd   = hd  Cmds,
            tag   = fst Cmd,
            param = snd Cmd
    in if tag = set then { change State }
            (pair ok nil)   :: (process param tl Cmds)
       else if tag = get then
            (pair st state) :: (process state tl Cmds)
       else (pair err nil)  :: (process state tl Cmds)

  in process state0 Cmds