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1630 :: How to generate {.gensym.} variable in a macro for a template?

• mora (orginal) [2015-09-12T13:05:30+02:00] view original

  Hi,

  I would like to have a macro which generates a variable within a template:

  template foo(): expr =
    var id {.gensym.} = 10
    id + 10
  
  echo foo() # echos 20
  echo foo() # echos 20
  
  import macros
  macro letsCreateVar(): stmt =
    result = parseExpr("var id {.gensym.} = 10")
  template foo2(): expr =
    letsCreateVar()
    id + 10
  
  echo foo2() # echos 20
  echo foo2() # error: redefination of 'id'
  Run

  It might seem like this has no real usage, the actual example is complicated (I'm happy to share if someone is interested). Is there a way to make this compile?

  Thank you,

  Peter

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• vega (orginal) [2015-09-13T05:38:41+02:00] view original

  Do you really need the variable?

  
  var tmp{.compiletime.} = 10
  
  proc foo(): int {.compiletime} =
    tmp = tmp + 10
    tmp
  
  echo foo() # echos 20
  echo foo() # echos 30
  Run

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• mora (orginal) [2015-09-13T21:40:02+02:00] view original

  I would like to print 20 every time. The value (10 in this case) and its type (int) is coming from the macro.

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• Araq (orginal) [2015-09-13T22:41:03+02:00] view original

  You can construct the AST via the API instead of hacking stuff together with parseExpr and then you can use macros.gensym.

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• jsanc (orginal) [2015-09-13T23:08:16+02:00] view original

  This works:

  import macros
  var defd {.compiletime.} = false
  macro letsCreateVar(): stmt =
    result =
      if defd: parseExpr("discard")
      else: parseExpr("var id {.gensym.} = 10")
    defd = true
  Run

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• mora (orginal) [2015-09-17T22:47:45+02:00] view original

  Hi! Thank you all the answers. I still couldn't generate the variable the way I wanted. Let me show the code: https://github.com/petermora/nimMapBenchmarks/blob/master/test12.nim

  In this example the templateF id is heavily overused: it is either a template if myMap12 is called with a lambda expression (for example proc (x: int): int = x+1). Also templateF can be a pointer to a proc (therefore a normal variable) if myMap12 is called with an already existing function.

  The code currently compiles. The problem is that if I call myMap12(someSeq, alreadyExistingFunction) twice, then the two templateF variables conflict.

  The motivation behind this strange code is that we could truly inline the given function if that is lambda.

  @Araq: To my excuse, I use parseExpr only here in the forum because I believe that it is more readable. I tried using macros.gensym, but I couldn't make it work.

  Peter

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