Re: can Python be useful as functional? (off topic)
On Tue, 18 Sep 2007 21:06:39 +0100, Jonathan Fine [EMAIL PROTECTED] wrote: Steve Holden wrote: You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. I've seen this before, and it is witty. However, it is perhaps unfair towards the woman. The man, after all, is someone who has offered a woman money in return for sex. The whole story reads differently if we replace 'philosopher' by 'man' and 'attractive lady' by 'woman'. The version I prefer has the woman asking, What sort of woman do you take me for? DaveM -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 9/18/07, Alex Martelli [EMAIL PROTECTED] wrote: Rustom Mody [EMAIL PROTECTED] wrote: 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 Change last line to for x in (x for x in si(l) if x % p != 0): yield x Thanks but why does (yield(x) for x in si(l) if x % p != 0) not work? I would have expected generator expression to play better with generators. More generally, if one wants to 'splice in' a generator into the body of a generator, is there no standard pythonic idiom? -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 18 Sep., 03:30, Evan Klitzke [EMAIL PROTECTED] wrote: My question is: how can we call a language functional if it's major implementation has a limited stack? Or is my code wrong? Python does not optimize tail recursion. Never mind. In the provided example the call to sieve() is not in tail position anyway ;) [...] If you want to write functional code, use a real functional language! It's hard to disagree. As a Python programmer I'd rather care for smooth integration with code written in Haskell or OCaml than adopting their particular programming idioms. For instance the Python - OCaml bridge is aged and I'm not aware that one between Python and Haskell even exists. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Rustom Mody [EMAIL PROTECTED] wrote: On 9/18/07, Alex Martelli [EMAIL PROTECTED] wrote: Rustom Mody [EMAIL PROTECTED] wrote: 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 Change last line to for x in (x for x in si(l) if x % p != 0): yield x Thanks but why does (yield(x) for x in si(l) if x % p != 0) not work? I would have expected generator expression to play better with generators. Why should it? It evaluates the expression which returns an object that just happens to be a generator and then as with any other expression that isn't assigned or returned it throws away the result. More generally, if one wants to 'splice in' a generator into the body of a generator, is there no standard pythonic idiom? Yes there is, as Alex showed you the standard python idiom for a generator to yield all elements of an iteratable (whether it is a generator or any other iterable) is: for somevar in iterable: yield somevar There have been various proposals in the past such as 'yield from iterable', but there doesn't seem any compelling case to introduce a new confusing syntax: the existing syntax works, and adding a special syntax wouldn't open the door to any performance benefits since the implementation would have to be pretty much the same (at most you would save a couple of local variable accesses). -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Lorenzo Stella a écrit : 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; Not exactly. They are used quite a lot, yes, but that's also the case in other paradigms. What's important in functional programming is *functions*. 2) any cycle in FP become recursion. FP idioms tends to use recursion instead of iteration, yes. But that's only viable with implementations doing tail-recursion optimisation - which is not the case with CPython (not that it couldn't FWIW - it's a design choice, and one of the few I don't necessarily agree with). I also know that Python got some useful tool such as map, filter, reduce... And all there itertools versions... so I told: let's try some FP-style programming with Python!. Most of the functional constructs that makes sens in Python are already idiomatic. And some common functional stuff are better reimplemented the pythonic way - as an example, while partial application is usually implemented with closures, and *can* indeed be implemented that way in Python, the class-based implementation is IMHO much better. 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? Strictly speaking, a language is functional if it has functions as first class objects. Period. According to this definition, Python is a functional language. Now that doesn't mean you should try to write Haskell in Python... IOW, your code is not wrong, but it's certainly not the best way to implement such an algorithm in Python. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Lorenzo Stella [EMAIL PROTECTED] writes: 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? It's no tthat it's wrong, but doing recursion in python can be problematic because there's no tail recursion optimisation and the size of the stack is limited (so eventually you'll run out of stack if you recurse deep enough). One way to capture the spirit of that Haskell program in Python is to use things from itertools; something like this (modified from http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/117119): import itertools def listprimes(n): def sieve(nums): seq = nums while True: prime = seq.next() seq = itertools.ifilter(prime.__rmod__, seq) yield prime if n == 0: return sieve(itertools.count(2)) else: return sieve(itertools.islice(itertools.count(2), n-1)) list(listprimes(100)) [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97] -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 18 Sep., 10:13, Bruno Desthuilliers bruno. [EMAIL PROTECTED] wrote: Lorenzo Stella a écrit : 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; Not exactly. They are used quite a lot, yes, but that's also the case in other paradigms. What's important in functional programming is *functions*. Functional lists are not quite the same. They are actually recursive datastructes. In Python you would model them as nested tuples: t = (a, (b, (c, ...(d, None) These are essentially pairs build from the bottom up using a list constructor and they have little in common with those mutable list objects ( arrays, vectors ) being used in Python. You can easily extend them into n-ary trees and implement mutations on them as forks where the original strucure is almost preserved. This leads to all kinds of functional data structures. In order to access an element you already need a recursive function defintion ( unless you just want to examine the head or the tail only ) and this makes functional programming and consed lists a perfect match. [...] Strictly speaking, a language is functional if it has functions as first class objects. Period. No, not period and not strictly speaking. A language is functional when its semantics is based on lambda calculus where everything is a function or a variable bound to a function which can be substituted by a function. Each functional language, to be usefull, must be augmented with programming language constructs used from other paradigms or support unsafe operations to enable proper side effects. This is not a humpty-dumpty issue where everyone can name his language a functional programming language just because one can pass functions as first class citizens around and he says so. Otherwise those languages can support a few functional programming language idioms such as map, reduce and filter or comprehensions as in Pythons case. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 2007-09-18, Kay Schluehr [EMAIL PROTECTED] wrote: On 18 Sep., 10:13, Bruno Desthuilliers bruno. [EMAIL PROTECTED] wrote: Lorenzo Stella a écrit : 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; Not exactly. They are used quite a lot, yes, but that's also the case in other paradigms. What's important in functional programming is *functions*. Functional lists are not quite the same. They are actually recursive datastructes. In Python you would model them as nested tuples: t = (a, (b, (c, ...(d, None) Tuples won't work for cyclic data, though. -- Neil Cerutti -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Kay Schluehr a écrit : On 18 Sep., 10:13, Bruno Desthuilliers bruno. [EMAIL PROTECTED] wrote: Lorenzo Stella a écrit : 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; Not exactly. They are used quite a lot, yes, but that's also the case in other paradigms. What's important in functional programming is *functions*. Functional lists are not quite the same. They are actually recursive datastructes. Linked lists, most of the time, yes. (snip) In order to access an element you already need a recursive function defintion ( unless you just want to examine the head or the tail only ) and this makes functional programming and consed lists a perfect match. Indeed. And that's also why some FP idioms don't translate directly in Python. [...] Strictly speaking, a language is functional if it has functions as first class objects. Period. No, not period and not strictly speaking. Ok, even on c.l.functional - where the above definition comes from BTW -, nobody really agree on the correct definition of functional !-) -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Lorenzo Stella wrote: [...] My question is: how can we call a language functional if it's major implementation has a limited stack? Or is my code wrong? So, which environment do you habitually use that provides an *unlimited* stack? You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. You just don't like the specific limit that Python imposes. So increase it with sys.setrecursionlimit(). regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden Sorry, the dog ate my .sigline -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 2007-09-18, Steve Holden [EMAIL PROTECTED] wrote: Lorenzo Stella wrote: [...] My question is: how can we call a language functional if it's major implementation has a limited stack? Or is my code wrong? So, which environment do you habitually use that provides an *unlimited* stack? Perhaps Lorenzo Stella is referring to Python's lack of tail-recursion optimization? There are languages that guarantee unlimited tail-recursion with a limited stack. That's a typical feature for a function language, right? You just don't like the specific limit that Python imposes. So increase it with sys.setrecursionlimit(). -- Grant Edwards grante Yow! I hope I bought the at right relish ... z visi.com... -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Steve Holden wrote: Lorenzo Stella wrote: .. So, which environment do you habitually use that provides an *unlimited* stack? You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. allegedly G B Shaw (http://findarticles.com/p/articles/mi_qn4158/is_19980925/ai_n14182408) -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Robin Becker [EMAIL PROTECTED] writes: Steve Holden wrote: Lorenzo Stella wrote: .. So, which environment do you habitually use that provides an *unlimited* stack? You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. allegedly G B Shaw (http://findarticles.com/p/articles/mi_qn4158/is_19980925/ai_n14182408) Also allegedly Winston Churchill, although wikiquote says: This is a very old joke where the participants vary dramatically from each telling. It's very unlikely though not impossible that the joke originated from Churchill. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Grant Edwards a écrit : On 2007-09-18, Steve Holden [EMAIL PROTECTED] wrote: Lorenzo Stella wrote: [...] My question is: how can we call a language functional if it's major implementation has a limited stack? Or is my code wrong? So, which environment do you habitually use that provides an *unlimited* stack? Perhaps Lorenzo Stella is referring to Python's lack of tail-recursion optimization? There are languages that guarantee unlimited tail-recursion with a limited stack. That's a typical feature for a function language, right? And also for some implementations of some purely procedural languages IIRC. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Paul Rudin wrote: Robin Becker [EMAIL PROTECTED] writes: Steve Holden wrote: Lorenzo Stella wrote: .. So, which environment do you habitually use that provides an *unlimited* stack? You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. allegedly G B Shaw (http://findarticles.com/p/articles/mi_qn4158/is_19980925/ai_n14182408) Also allegedly Winston Churchill, although wikiquote says: This is a very old joke where the participants vary dramatically from each telling. It's very unlikely though not impossible that the joke originated from Churchill. Also allegedly Bertrand Russell, who was going to be the subject of my version until I realized that I would get many corrections to any asserted identity of the originator. I should have know to expect just as many corrections to the absence of any such assertion, this being c.l.py :-) regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden Sorry, the dog ate my .sigline -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional? (off topic)
Steve Holden wrote: You remind me of the conversation between the philosopher and an attractive lady whom he was seated next to at dinner. He asked her if she would sleep with him for a million dollars, to which she readily agreed. So he followed this by asking her if she'd sleep with him for a dollar. She replied: No. Do you take me for a prostitutte?, to which his riposte was We have already established that fact, and are now merely haggling about the price. I've seen this before, and it is witty. However, it is perhaps unfair towards the woman. The man, after all, is someone who has offered a woman money in return for sex. The whole story reads differently if we replace 'philosopher' by 'man' and 'attractive lady' by 'woman'. -- Jonathan -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
On 18 Set, 18:51, Grant Edwards [EMAIL PROTECTED] wrote: Perhaps Lorenzo Stella is referring to Python's lack of tail-recursion optimization? There are languages that guarantee unlimited tail-recursion with a limited stack. That's it. Rustom Mody: your implementation lacks exactly where mine does. Try listing the first 2000 primes... That's what I meant: I cannot in general (with Python) get such list just by defining *what* it is, I have to express *how* to get it (describing an algorithm). What or How: that is the question. Steve Holden wrote: You just don't like the specific limit that Python imposes. So increase it with sys.setrecursionlimit(). That is obviously not the answer to my question. -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Rustom Mody asked: [...] why does (yield(x) for x in si(l) if x % p != 0) not work? I would have expected generator expression to play better with generators. You have a statement, yield(x), where the construct requires an expression. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
can Python be useful as functional?
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
Re: can Python be useful as functional?
On 9/17/07, Lorenzo Stella [EMAIL PROTECTED] 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? Python does not optimize tail recursion. You can increase the maximum recursion limit with sys.setrecursionlimit, but the code will still be slow. I am a fan of functional programming languages (including Haskell!), but I wouldn't try to write functional code in Python -- the language isn't optimized for this type of code, and the syntax it provides isn't very elegant, compared to other functional languages. If you want to write functional code, use a real functional language! -- Evan Klitzke [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
Rustom Mody [EMAIL PROTECTED] wrote: 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 Change last line to for x in (x for x in si(l) if x % p != 0): yield x if you wish. Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: can Python be useful as functional?
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 [EMAIL PROTECTED] 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 -- http://mail.python.org/mailman/listinfo/python-list
