Comprehensions in Python and Javascript are only very basic?

Question

Looking at comprehensions in Python and Javascript, so far I can't see some of the main features that I consider most powerful in comprehensions in languages like Haskell.

Do they allow things like multiple generators? Or are they just a basic map-filter form?

If they don't allow multiple generators, I find them quite disappointing - why have such things been left out?

Answer 1

Yes, you can have multiple iterables in a Python list comprehension:

>>> [(x,y) for x in range(2) for y in range(3)]
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2)]

Answer 2

Python allows multiple generators:

>>> [(x,y,x*y) for x in range(1,5) for y in range(1,5)]
[(1, 1, 1), (1, 2, 2), (1, 3, 3), (1, 4, 4), 
 (2, 1, 2), (2, 2, 4), (2, 3, 6), (2, 4, 8), 
 (3, 1, 3), (3, 2, 6), (3, 3, 9), (3, 4, 12),
 (4, 1, 4), (4, 2, 8), (4, 3, 12), (4, 4, 16)]

And also restrictions:

>>> [(x,y,x*y) for x in range(1,5) for y in range(1,5) if x*y > 8]
[(3, 3, 9), (3, 4, 12), (4, 3, 12), (4, 4, 16)]

Update: Javascript's syntax is similar (results from using the javascript shell on firefox):

var nums = [1, 2, 3, 21, 22, 30];
var s = eval('[[i,j] for each (i in nums) for each (j in [3,4]) if (i%2 == 0)]');
s.toSource();
[[2, 3], [2, 4], [22, 3], [22, 4], [30, 3], [30, 4]]

(For some reason, something about the context stuff is evaluated in in the javascript shell requires the eval indirection to have list comprehensions work. Javascript inside a <script> tag doesn't require that, of course)

Answer 3

Add an if statement as well...

>>> [(x,y) for x in range(5) for y in range(6) if x % 3 == 0 and y % 2 == 0]
[(0, 0), (0, 2), (0, 4), (3, 0), (3, 2), (3, 4)]

Answer 4

Comprehensions is very powerful in Haskell to a large extent because Haskell is functional, so it makes extremely much sense for them to be. Python is not functional so it makes less sense.

You can make a lot of complex things with comprehensions in Python but it quickly becomes hard to read, thereby defeating the whole purpose (meaning you should do it some other way).

However, as pointed out here, python does allow multiple generators in comprehensions.