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Comment by LeoPanthera | Hacker News Reader

I love Python's list comprehensions. When I first discovered them I had a kind of "mind blown" moment. They look like this, for anyone who doesn't know:

  squared_div_by_3 = [i**2 for i in range(10) if i % 3 == 0]

Without using a list comprehension, this is equivalent to:

  squared_div_by_3 = []
  for i in range(10):
      if i % 3 == 0:
          squared_div_by_3.append(i**2)

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Of course, Python would have been way more awesome if they did not reject the much more reader-friendly and idiomatic:

  squared_div_by_3 = [
      for i in range(10):
          if i % 3 == 0:
              i**2
  ]

That is, just transforming the normal for loop and if into a list comprehension by surrounding it with brackets. It would even preserve the friendly forced blocking and indentation. Can you imagine?

It's inspired by set-builder notation: https://en.wikipedia.org/wiki/Set-builder_notation

Nim has a `collect` macro which works exactly like this.

Reads less like Perl in Lisp:

    (loop for i below 10 when (zerop (mod i 3)) collect (* i i))

Are you joking?

With iterate, for comparison:

  (iter (for i below 10) (if (zerop (mod i 3)) (collect (* i i))))

There seems to have been a design emphasis in C# on providing something similar under the name LINQ. It would look like this:

    var squared_div_by_3 = from i in Enumerable.Range(0,10) where i % 3 == 0 select i*i;

or alternatively:

    var squared_div_by_3 = Enumerable.Range(0,10).Where(i => i % 3 == 0).Select(i => i*i);

In that second syntax, `Where` is exactly equivalent to Python's `filter` (or lisp's `remove-if-not`) and `Select` is of course equivalent to `map`; I assume the odd choice of names for C# is meant to appear similar to SQL.

They're introducing Range objects with syntactic sugar so you can say `0..10` instead of `Enumerable.Range(0,10)`, and you'd think there would be LINQ implementations for those objects, but there aren't.

In Haskell:

let xs = [x*x | x <- [0..], x `mod` 3 == 0]

You can decide later how many you actually want.

In racket:

    (for/list ([i (in-range 10)
               #:when (zero? (remainder i 3))])
       (pow i 2))

there’s also for/vector for/and for/or for/first for/last, most likely for/set. Hashes. can add your own, etc…

I've not used python much but is that not the same/similar to the unfold function in functional languages?

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