can Python be useful as functional?

Rustom Mody rustompmody at gmail.com
Tue Sep 18 07:12:00 CEST 2007


The following defines the infinite list of primes as a generator [chap
6.5 of the library]

def sieve(l):
    p = l.next()
    yield p
    for x in sieve(l):
        if x % p != 0:
            yield x

After that

from itertools import *
>>> [p for i,p in izip(range(10), sieve(count(2)))]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
>>>


I tried to write a shorter generator expression based sieve but cant
get it right.
Can someone help? Heres the non-working code

def si(l):
    p = l.next()
    yield p
    (x for x in si(l) if x % p != 0)

There should be an yield or return somewhere but cant figure it out

On 9/18/07, Lorenzo Stella <lorestar at gmail.com> wrote:
> Hi all,
> I haven't experienced functional programming very much, but now I'm
> trying to learn Haskell and I've learned that: 1) in functional
> programming LISTS are fundmental; 2) any "cycle" in FP become
> recursion.
> I also know that Python got some useful tool such as map, filter,
> reduce... so I told: "let's try some FP-style programming with
> Python!". I took a little example of Haskell:
>
>      listprimes :: Integer -> [Integer]
>      listprimes n = if n == 0 then sieve [2..] else sieve [2..(n-1)]
> where
>             sieve [] = []
>             sieve (p:xs) = p : sieve (filter (\x -> mod x p > 0) xs)
>
> and I tried to "translate" it in Python:
>
>      def sieve(s):
>          if s == []:
>              return []
>          else:
>              return [s[0]] + sieve(filter((lambda x: x % s[0] > 0),
> s[1:]))
>
>      def listprimes(n):
>          return sieve(range(2,n))
>
> These should be almost the same: listprimes actually lists prime
> integers up to n-1. The result is: Haskell implementation works well,
> maybe it's not the better way to do it, but it does what I wanted.
> Python implementation gives me
>
>      RuntimeError: maximum recursion depth exceeded in cmp
>
> My question is: how can we call a language "functional" if it's major
> implementation has a limited stack? Or is my code wrong?
>
> LS
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>



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