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

Your problem has been had before: Recursive declaration of function pointer in C

C++ operator overloading can be used to hide the mechanics of what is essentially the same as your your C and Haskell solutions, as Herb Sutter describes in GotW #57: Recursive Declarations.

struct FuncPtr_;
typedef FuncPtr_ (*FuncPtr)();

struct FuncPtr_
{
  FuncPtr_( FuncPtr pp ) : p( pp ) { }
  operator FuncPtr() { return p; }
  FuncPtr p;
};

FuncPtr_ f() { return f; } // natural return syntax

int main()
{
  FuncPtr p = f();  // natural usage syntax
  p();
}

But this business with functions will, in all likelihood, perform worse than the equivalent with numeric states. You should use a switch statement or a state table, because what you really want in this situation is a structured semantic equivalent to goto.