Nested array comprehensions in CoffeeScript

Question

In Python

def cross(A, B):
   "Cross product of elements in A and elements in B."
   return [a+b for a in A for b in B]

returns an one-dimensional array if you call it with two arrays (or strings).

But in CoffeeScript

cross = (A, B) -> (a+b for a in A for b in B)

returns a two-dimensional array.

1. Do you think it's by design in CoffeeScript or is it a bug?
2. How do I flatten arrays in CoffeScript?

Answer 1

First I would say say that 2 array comprehensions in line is not a very maintainable pattern. So lets break it down a little.

cross = (A, B) ->
  for a in A
    for b in B
      a+b

alert JSON.stringify(cross [1,2], [3,4])

What's happening here is that the inner creates a closure, which has its own comprehension collector. So it runs all the b's, then returns the results as an array which gets pushed onto the parent comprehension result collector. You are sort of expecting a return value from an inner loop, which is a bit funky.

Instead I would simply collect the results myself.

cross = (A, B) ->
  results = []
  for a in A
    for b in B
      results.push a + b
  results

alert JSON.stringify(cross [1,2], [3,4])

Or if you still wanted to do some crazy comprehension magic:

cross = (A, B) ->
  results = []
  results = results.concat a+b for b in B for a in A
  results

alert JSON.stringify(cross [1,2], [3,4])

Whether this is a bug in CS or not is a bit debatable, I suppose. But I would argue it's good practice to do more explicit comprehension result handling when dealing with nested iterators.

Answer 2

https://github.com/jashkenas/coffee-script/issues/1191