Haskell to Python: multiple-functions conversion issue

Question

I am pretty new to programming and I was asked to convert 3 haskell functions into python as a practice exercise. The 3 functions are connected, since output of one is used as the input of the next one and so on.

I get what the haskell functions do, but I have no idea how to start converting them!

This is the haskell code:

factorial :: Int -> Int
factorial n = product (down n)

product :: [Int] -> Int
product [] = 1
product (a:as) = a * product as

down :: Int -> [Int]
down 0 = []
down n = n : down (n-1)

And this was my try of converting it:

class function:
    def down(self):
        if self.n == 0:
            self.lista = []
        else:
            self.lista = range(self.n, 0, -1)

    def product(self):
        for x in self.lista:
            if x == []:
                self.product = 1
            if x != []:
                for i in self.lista:
                    self.product = i * self.product

    def factorial(self):
        self.n = int(raw_input("n="))

        self.fact = self.product(self.down(self.n))

        print self.fact
        raw_input()

c = function()
c.factorial()

Well, first I believe this isn't a 'direct conversion' of the haskell code. That would be okay though, but second, it doesn't work.

This lack of programming background is killing me...can anyone help me with this problem?

Thanks a lot!

Edit:

The point of this issue is to exactly convert the haskell into python. I did a stripped version of it myself, it was the next step in the exercise ^^

Converting code from one language you don't fully understand to another language you don't fully understand is a terrible way to learn programming. I would suggest to whoever is giving you exercises that they get you to the point of feeling comfortable reading and writing code in one language first. — Ben
– Ben, Commented Nov 2, 2012 at 2:51

Deestan · Accepted Answer · 2012-11-01 17:58:29Z

7

First off, ditch the class wrapper - that's not needed.

A straight Python translation would be something like:

# factorial :: Int -> Int
def factorial(n):
    return product(down(n))

# product :: [Int] -> Int
def product(arr):
    if len(arr) == 0: return 1
    a, ar = arr[0], arr[1:]
    return a * product(ar)

# down :: Int -> [Int]
def down(n):
    if n == 0: return []
    return [n] + down(n - 1)

But the recursive style is not very Pythonic here. Next exercise: replace recursion with iteration, list comprehensions, or calls to reduce (if on Python2).

answered Nov 1, 2012 at 17:58

Deestan

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6 Comments

mgilson Over a year ago

final exercise, read about the factorial function in the math module in the stdlib :D

Edward Coelho Over a year ago

Oh, I am amazed at how simply this could be! Had no idea haskell was so similar to python. Thanks a lot!

user4815162342 Over a year ago

The problem with this code is not that it's unpythonic (i.e. not idiomatic Python), but that it gives the beginner a false impression of similarity between Python and Haskell. Python's lists are contiguous arrays, so getting the remaining members is an expensive operation for long lists. In other words, Python's ar = arr[1:] is quite different in terms of CPU and memory usage from Haskell's unpacking of first and remaining list elements.

Edward Coelho Over a year ago

Ugh, but now how do I make the variables pass from one function to another? This is my first time working with multiple functions, that's why I used the class wrapper ^^'

Deestan Over a year ago

@EdwardCoelho Same way as they do in the Haskell source, they are given as parameters in the function calls. :)

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user4815162342 · Accepted Answer · 2012-11-01 17:53:42Z

4

If you want to write idiomatic Python, avoid recursion.

down(n) is spelled as range(n, 0, -1). Use xrange if you want lazy semantics, which would be closer to the spirit of Haskell.

product(lst) is reduce(operator.mul, lst, 1). (It would be more idiomatic to just spell out the loop, but this is shorter.)

From there on it should be obvious how to convert factorial.

answered Nov 1, 2012 at 17:53

user4815162342

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7 Comments

Edward Coelho Over a year ago

Wow thanks! I really should be using lazy semantics and didn't know about that!

user4815162342 Over a year ago

If you like lazy semantics, you'll love generators. Python is lazier than people tend to think. :)

user4815162342 Over a year ago

@EdwardCoelho You're welcome. If you found the answer useful, feel free to click the "accept" button. :)

Edward Coelho Over a year ago

I certainly did! I'm just trying to finishing the code. I always do that before accepting :p

Ben Over a year ago

If you like lazy semantics and to be able to use values more than once, you'll hate generators.

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