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

In the C-like type systems functions are not first order citizens. There are certain restrictions on handling them. That was a decision for simplicity and speed of implementation/execution that stuck. To have functions behave like objects, one generally requires support for closures. Those however are not naturally supported by mosts processors' instruction sets. As C was designed to be close to the metal, there was no support for them.

When declaring recursive structures in C, the type must be fully expandable. A consequence of this is, that you can only have pointers as self-references in struct declarations:

struct rec;
struct rec {
    struct rec *next;
};

Also every identifier we use has to be declared. One of the restrictions of function-types is, that one can not forward declare them.

A state machine in C usually works by making a mapping from integers to functions, either in a switch statement or in a jump table:

typedef int (*func_t)();

void run() {
    func_t table[] = {a, b, c};

    int state = 0;

    while(True) {
        state = table[state]();
    }
}

Alternatively you could profile your Python code and try to find out why your code is slow. You can port the critical parts to C/C++ and keep using Python for the state machine.