On Tue, 08 Feb 2005 17:36:19 +0100, Bernhard Herzog wrote: > Nick Vargish <[EMAIL PROTECTED]> writes: > >> "Xah Lee" <[EMAIL PROTECTED]> writes: >> >>> is it possible to write python code without any indentation? >> >> Not if Turing-completeness is something you desire. > > It's possible to implement a turing machine with a single list > comprehension. No indentation needed.
I had to think about it, it's an interesting, and I'm going to tentatively disagree, because of the statement/expression dichotomy. "Tentatively" because if somebody can answer these objections than I will happily change my mind :-) I can't figure out how to write a TM in a Python List Comprehension without one of either "variable binding" (so we can store the last symbol list and manipulate it in the next iteration) or "recursive function" (to express the whole tape as a recursive function), both of which require statements. I can figure out how to write a single state transition, but in a single LC I can't figure out how to feed the new state into the next iteration; the previous values generated in the LC are, to my knowledge, not accessible to the LC as it is running. (If they are, I *think* that would indeed be enough.) I'm sure Haskell could do both, being a functional language, and I am satisfied that it is probably possible in that language, as long as you are resigned to creating a wasted list (which may not even matter in Haskell). But I think you're screwed in Python with an LC. However, hack hack hack, I think you could do this in Python 2.4 with a *generator* comprehension and a couple of supporting lines of non-indented code: Python 2.4 (#1, Jan 2 2005, 22:17:50) [GCC 3.4.3 (Gentoo Linux 3.4.3, ssp-3.4.3-0, pie-8.7.6.6)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import sys >>> this = sys.modules[__name__] >>> magicGenerator = (getattr(this, "results")[-1] + 1 for x in range(5)) >>> results = [0] # you have to prime the state here >>> results.extend(magicGenerator) >>> results [0, 1, 2, 3, 4, 5] >>> Now you *can* get at the previous state and write a state-transition expression in perfectly legal Python. What do you know, generator comprehensions are Turing Complete and list comprehensions aren't. I wouldn't have expected that. People caught using this technique in real code will be caught and forced to code in Intercal for the rest of their lives. -- http://mail.python.org/mailman/listinfo/python-list
