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141 :: scope guards

• Jason (orginal) [2013-04-07T15:59:27+02:00] view original

  I'm checking out Nimrod, and I like what I see alot.

  One thing I thought was brilliant in D was the scope(exit) statement for simple and readable resource management (cleaner than RAII). Is there a way to do the same thing in Nimrod?

  If not built in, perhaps with a macro? Scope statements make it much easier to write exception safe code. It is simply syntactic sugar, but that sugar is very sweet.

  
  {
    statements1
    scope(exit) statement2
    statements3
  }
  Run

  is transformed or lowered into:

  
  {
    statements1
    try {
       statements3
    } finally {
       statement2
    }
  }
  Run

  The real benefit comes when you have to use many scope statements in a function to manage several resources, because it converts very nested code into straight line code. See The D Programming Language, pages 84-87 for discussion (the above transformation should be credited to those pages, which I am quoting from directly). I see that Nimrod has several macro features, would it be easy to use them to implement such a feature?

  Thank you.

  Best regards,

  Jason

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• gradha (orginal) [2013-04-07T19:02:36+02:00] view original

  You can certainly do that as long as you do single statements. See the Except and finally statements in the manual. Presumably working nimrod code matching your example would be:

  statements1
  finally: statement2
  statements3
  Run

  At the moment except: can't be used with a specific exception, it has to be a catch all.

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• Jason (orginal) [2013-04-07T22:47:55+02:00] view original

  Sweet. In case future readers have this same question, I will demonstrate the lovely behavior of nimrod with respect to multiple finally statements. This is elegant. Thank you @gradha.

  
  #!/usr/bin/env nimrod c -r
  
  proc testFinally(): void =
      echo("-- test 1")
      echo("a")
      finally: echo("e")
      echo("b")
      finally: echo("d")
      echo("c")
  
  testFinally();
  # expect: "abcde"
  
  
  proc testFinally2(): void =
      echo("-- test 2")
      echo("a")
      finally: echo("b")
      if true: raise newException(EArithmetic, "some exception")
      echo("c")
      finally: echo("e")
      echo("d")
  
  
  proc call2() =
      try: testFinally2();
      except: nil
  
  call2()
  # expect: "ab"
  
  
  proc testFinally3(): void =
      echo("-- test 3")
      echo("a")
      finally: echo("d")
      echo("b")
      finally: echo("c")
      if true: raise newException(EArithmetic, "some exception")
      echo("e")
  
  proc call3() =
      try: testFinally3();
  
  call3()
  # expect: "abcd"
  
  Run

  beatifully produces:

  
  -- test 1
  a
  b
  c
  d
  e
  -- test 2
  a
  b
  -- test 3
  a
  b
  c
  d
  Run

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• dom96 (orginal) [2013-04-07T23:24:12+02:00] view original

  Some suggestions in case you're not aware:

  You can safely omit the : void from proc definitions, and Nimrod doesn't require you to end your lines with a semicolon ;)

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• gradha (orginal) [2013-05-01T21:52:53+02:00] view original

  Note that if you use except/finally as a statement your code will do unexpected things if you try to concatenate it into a single line with a semicolon (see https://github.com/Araq/Nimrod/issues/412). But you can write multiple statements in a block without problems.

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• timothee (orginal) [2018-03-29T22:55:03+02:00] view original

  Note that if you use except/finally as a statement your code will do unexpected things if you try to concatenate it into a single line with a semicolon (see https://github.com/Araq/Nimrod/issues/412).

  this bug has been fixed https://github.com/nim-lang/Nim/issues/412

  Presumably working nimrod code matching your example would be: ...

  IIRC now we use defer: instead of finally: for that although both seem to work. see also: https://github.com/timotheecour/D_vs_nim/pull/20#issuecomment-377364528

  EDIT: standalone finally is deprecated: Warning: use 'defer'; standalone 'finally' is deprecated [Deprecated]

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• mratsim (orginal) [2018-03-30T12:27:08+02:00] view original

  It's a post from 2013 ;).

  Anyway the idiomatic way to do scope guard is with a template that does try: body finally: as it's done by Udiknedormin here.

  There is an example in the guards and lock section of the manual too.

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