Saturday, September 14, 2013

Pythlog, assignments, and equations

Pythlog is a language I'm working on from time to time. It's a dialect of Python that incorporates constraint logic programming. So far it supports integers, list, tuples, strings, and user defined classes. There are a lot things still missing, but it already capable of some pretty cool stuff:
    def fac(n):  
        assert n >= 0  
        if n == 0:  
            return 1  
            return fac(n - 1) *  n

    print(fac(7)) # Prints 5040
    fac(w) = 5040
    print(w) # Prints 7
In this example we define the factorial function the a pretty straight-forward recursive manner. Then we call it to calculate the factorial of 7. The second to last line might appear a bit unorthodox though. What's going on here?

As said, Pythlog is a logic programming language. Such languages have a few features that set the aside from traditional imperative languages. On such feature is so call free variable. The code above is equivalent to:
    w = free
    assert fac(w) == 5040
where w is a free variable which is passed to fac. By asserting that the return value must be equal to 5040, the constraint satisfaction framework kicks in and solves w to 7.

I recently introduced the shorthand syntax fac(w) = 5040 for this. I'm not fully happy with it yet, because there are some non-obvious behaviors. I'm pretty sure there will be some changes in this area. For now though, it make the language at least look nicer, soon I hope it will also feel nicer.