Clean and type-safe state machine implementation in a statically typed language?

Question

I implemented a simple state machine in Python:

import time

def a():
    print \"a()\"
    return b

def b():
    print \"b()\"
    return c

def c():
    print         


        

Answer 1

The problem with your Haskell code is, that type only introduces a synonym, which is quite similar to what typedef in C does. One important restriction is, that the expansion of the type must be finite, you can't give a finite expansion of your state machine. A solution is using a newtype: A newtype is a wrapper that does only exist for the type checker; there is absolutely zero overhead (excluded stuff that occurs because of generalization that can't be removed). Here is your signature; it typechecks:

newtype FN = FN { unFM :: (IO FN) }

Please note, that whenever you want to use an FN, you have to unpack it first using unFN. Whenever you return a new function, use FN to pack it.