Nim equivalent of Python's list comprehension

Question

Since Nim shares a lot of features with Python, i would not be surprised if it implements Python's list comprehension too:

string = "Hello 12345 World"
numbers = [x for x in string if x.isdigit()]
# ['1', '2', '3', '4', '5']

Is this actually possible in Nim? If not, could be implemented with templates/macros ?

Endocardium

Answer 1

UPDATE: List comprehension has been deprecated since version 0.19.9 (Source). A good alternative is to use the new sugar.collect macro.

Another update: As of 2023, list comprehension has been removed.

---

Outdated original answer

List comprehension is implemented in Nim in the sugar package (i.e., you have to import sugar). It is implemented as a macro called lc and allows to write list comprehensions like this:

lc[x | (x <- 1..10, x mod 2 == 0), int]

lc[(x,y,z) | (x <- 1..n, y <- x..n, z <- y..n, x*x + y*y == z*z), tuple[a,b,c: int]]

Note that the macro requires to specify the type of the elements.

Excurrent

@Downvoter: Surprised to get downvotes here. Is it just because the other answer has been updated or is there something wrong with my answer? – Excurrent

Your answer is right, and since you were the first that commented on future module, i'll mark yours as the correct. – Endocardium

List comprehensions are deprecated since version 0.19.9. – Wanonah

For a library-based solution check out github.com/alehander42/comprehension – Bragdon

the prefered way of doing this is with the collect macro in sugar nim-lang.org/docs/sugar.html#collect.m%2Cuntyped%2Cuntyped – Glabrous

@daten-kieker That information that the answer is outdated is written in the first paragraph of the answer. I'll try to make it even more obvious... – Excurrent

@Excurrent - sorry, my bad. Too focused on the code snippet, I guess. I'll delete my comment. – Mischief

Answer 2

According to rosettacode, Nim has no list comprehensions, but they can be created through metaprogramming.

[EDIT]

As pointed out by bluenote10, list comprehensions are now part of the future module:

import future
var str = "Hello 12345 World"
echo lc[x | (x <- str, ord(x) - ord('0') in 0..9),  char]

The above snippet yields @[1, 2, 3, 4, 5]

Azzieb

Looks like this is exactly what is now part of the future module. – Excurrent

As of 2023-06-17 this answer doesn't work anymore. The new way is the answer by @dotfelixb) – Mischief

Answer 3

Original

import sugar

let items = collect(newSeq):
  for x in @[1, 2, 3]: x * 2

echo items

outputs @[2, 4, 6]

Updated to answer question

import sugar
import sequtils
import strutils

let numbers = collect: # collect from suger
  for x in "Hello 12345 World".toSeq: # toSeq from sequtils
    if x.isDigit: # isDigit from strutils
      x

echo numbers

outputs @['1', '2', '3', '4', '5']

Palmira

Answer 4

Nim translation of your code:

import sugar, strutils
var str = "Hello 12345 World"
echo collect(for s in str:
  if s.isDigit(): s)

Aegisthus