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10504 :: for loop iteration variables

• jhgalino (orginal) [2023-09-21T10:47:51+02:00] view original

  I converted a golang code snippet from https://go.dev/blog/loopvar-preview to nim.

  var prints: seq[proc ()] = @[]
  for i in 1..3:
    prints.add(proc () = echo i)
  
  for print in prints:
    print()
  Run

  This code currently prints 3, 3, 3. How do I modify the code so that the output would be 1, 2, 3?

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• enthus1ast (orginal) [2023-09-21T10:54:43+02:00] view original

  You need to use "closureScope"

  https://nim-lang.org/docs/system.html#closureScope.t%2Cuntyped

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• janAkali (orginal) [2023-09-21T11:30:52+02:00] view original

  Note the note under the 'closureScope':

  Note: This template may not work in some cases, use capture instead.

  See capture template in std

  And here's a working code snippet:

  import sugar
  
  var prints: seq[proc()]
  for i in 1..3:
    capture i:
      prints.add(proc() = echo val)
  
  for print in prints:
    print()
  Run

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• jhgalino (orginal) [2023-09-21T11:32:46+02:00] view original

  Thanks! The capture macro did it.

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• enthus1ast (orginal) [2023-09-21T13:58:59+02:00] view original

  interesting

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• Difget (orginal) [2023-09-25T11:48:50+02:00] view original

  Great job on converting the Go code to Nim! To modify the Nim code so that it prints 1, 2, 3, you can capture the value of i for each iteration using a temporary variable. Here's the modified code:

  nim Copy code var prints: seq[proc ()] = @[]

  for i in 1..3:

  let tmp = i prints.add(proc () = echo tmp)

  for print in prints:

  print()

  By introducing the tmp variable and capturing the value of i within each iteration, you ensure that each closure in the prints sequence has its own copy of i, resulting in the desired output of 1, 2, 3.

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• treeform (orginal) [2023-09-25T17:32:47+02:00] view original

  What are pros and cons of altering the Nim language to always capture the for loop variable like go did? Will it make code that much slower? Most of the time capture for loop variable does not matter and should not impact performance?

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• Araq (orginal) [2023-09-25T17:45:10+02:00] view original

  Nim started with doing the right thing but we changed it to the wrong thing so that JS interop becomes easier and in order to speed up callback-heavy code (cough async).

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