Pattern matching function declarations in Python part II

Ok, so the pattern matching idea kind of backfired.

I was under the impression that a function's default arguments wouldn't be evaluated until the function was called. Which would have meant that I could've changed them, into valid Python code, in the decorator function. But... that's not the way it works.

My first attempt at something more interesting than matching a constant failed miserably. I wanted to use a variable to constrain that matching process. For example:

>>> @patternmatched
... def foo(lst = [__, var, var, __]):
...   pass

should match any lst which is a list and where there is, at least one, repeated element somewhere in it. Both [1,1,2,3,4] and [3,2,1,1,2] should match, whilst [1,2,1,3,1] shouldn't.

Unfortunately. Trying the above results in a NameError since both __ and var are unbound. And, seeing that, I remembered that this is *exactly* what happened about three years ago when I was trying out various syntaxes for PyRete. It's the reason I decided to put all of the matching in the if-statement instead of in the function arguments.

But, it's not quite time yet to give up. There are several possible ways to go about this. The "best" choice is probably to use Andrew's python4ply, or maybe PyPy. Either way, I'll get a chance to compare and contrast the work of implementing a DSL in Python versus Common Lisp.

Pattern matching function declarations in Python

All those who have tried to write a rules-based program know that there are, at least, two differences between "regular" programming and rules-based programming. The first is that you have no direct control over the flow of execution and the second is that, instead of specifying input parameters, you specify patterns that should be matched for a certain "function" (or rule) to be invoked.

The bit about giving up control is quite difficult for most programmers (at least the ones I know) and it doesn't translate well to other types of programming anyway. But it would be interesting to try and add pattern matching functions to a language such as Python and see whether it could be useful (or at least fun to experiment with).

It's not a new idea. The first language to support pattern matching this way was Snobol (version 4 I think) which was introduced in the late 60s/early 70s. Both Haskell and Erlang has it and there are a bunch of others as well, Qi and Prolog to mention a few.

I'm thinking about something along the lines of:

>>> from patternmatching import *
>>> @patternmatched
... def fac(n = 0):
...   return 1
...
>>> @patternmatched
... def fac(n = int):
...   return n* fac(n = n-1)
...
>>> fac(5)
120
>>>

The idea is that the first function would handle any calls to the fac function where the parameter n is 0 and the second function would handle any calls where the parameter n is an int (but not 0). And, yes. I've got the above working already. The tricky bits are providing more elaborate forms of pattern matching with the available syntax.

I'll be back shortly with some code...