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3193 :: lambda capture in Nim

• woggioni (orginal) [2017-09-23T19:43:39+02:00] view original

  As far as I could understand this Nim code

  
  var arr = newSeq[proc()]()
  for j in 0..<10:
      let x : int = j + 1
      let p = proc() = echo x
      arr.add(p)
  
  for p in arr:
      p()
  
  Run

  should be equivalent to the following Javascript code

  
  var arr = []
  for (let j=0; j<10; j++)
  {
      let x = j + 1
      let p = () => console.log(x)
      arr.push(p)
  }
  for(p of arr) p()
  Run

  actually the Nim version always prints 10 instead of the whole sequence from 1 to 10.. I cannot understand how lambda capture works since the captured variable x should be local to the scope of the for loop and a new instance should be created for every iteration. Also, is there a way to capture variables by value? How could I fix the previous example to make it functionally equivalent to the Javascript one?

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• gokr (orginal) [2017-09-23T20:33:32+02:00] view original

  This works:

  
  var arr = newSeq[proc()]()
  for j in 0..<10:
    closureScope:
      let x : int = j + 1
      let p = proc() = echo x
      arr.add(p)
  
  for p in arr:
    p()
  
  
  Run

  See https://nim-lang.org/docs/manual.html#closures-creating-closures-in-loops

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• woggioni (orginal) [2017-09-24T11:32:56+02:00] view original

  Thanks, I had missed that paragraph in the manual! So without the closureScope template the loop just generates one closure with the intances used in the last iteration and every proc that tries to make a capture actually captures that closure, is that correct?

  Also, it is totally unrelated, but is there a specific reason why I cannot compile the generated Javascript source with the closure compiler?

  It gives me an error because of an undeclared variable in a loop

  
  nimcache/jpacrepo.js:1727: ERROR - variable property is undeclared
                          for (property in x_147603) {
  
  Run

  simply replacing "property" with "var property" seems to fix the issue (the variable 'property' seems to be used only within the scope of the for loop, it's not used as a global variable, so this should be safe to do)

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