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4667 :: Note how the ``let fn = filename`` statement ensures that ``filename`` is evaluated only once

• greenfork (orginal) [2019-02-19T17:41:27+01:00] view original

  The title is the last sentence from the https://nim-lang.org/docs/tut2.html#templates and it refers to the following snippet of code:

  template withFile(f: untyped, filename: string, mode: FileMode,
                    body: untyped): typed =
    let fn = filename
    var f: File
    if open(f, fn, mode):
      try:
        body
      finally:
        close(f)
    else:
      quit("cannot open: " & fn)
  
  withFile(txt, "ttempl3.txt", fmWrite):
    txt.writeLine("line 1")
    txt.writeLine("line 2")
  Run

  I can see fn is used in if open(f, fn, mode): and in quit("cannot open: " & fn). But why would it be evaluated twice if we did not use let? Aren't arguments for template immutable?

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• Stefan_Salewski (orginal) [2019-02-19T19:14:04+01:00] view original

  Interesting question. We can easily prove that the statement from tutorial is correct:

  Run

  proc str: string =

  echo "str proc called" "strprocresult"

  template logTwice(msg: string) =

  #let t = msg #echo t #echo t echo msg echo msg

  proc main =

  logTwice(str())

  main()

  
  $ ./t
  str proc called
  strprocresult
  str proc called
  strprocresult
  
  Run

  So indeed not a plain string is passed to the template, but the passed expression is called inside of template. I think for typed or untyped arguments we may expect that, but maybe not for real data types like string? I guess one of the Nim devs will write some words of explanations later.

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• cdunn2001 (orginal) [2019-02-19T19:33:42+01:00] view original

  If open fails, the else block is called, in which case filename would be evaluated again.

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• Araq (orginal) [2019-02-19T20:25:10+01:00] view original

  Templates use a simple substitution mechanism, besides the fact that you don't have to care about putting your arguments in () (see C's preprocessor), it's quite comparable to what C's preprocessor does.

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• greenfork (orginal) [2019-02-20T08:24:11+01:00] view original

  Templates use a simple substitution mechanism, besides the fact that you don't have to care about putting your arguments in () (see C's preprocessor), it's quite comparable to what C's preprocessor does.

  That makes sense, thanks!

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