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866 :: "Conditional let" problem

• AxBen (orginal) [2015-02-13T13:30:19+01:00] view original

  How would I make this work (right now nim gives "Error: undeclared identifier: 'yygnu'", a case which could be defended, given that the compiler has to go a long way to find, that if it reaches "cp = yygnu..." yygnu is either defined or an error has been thrown)?

  import os
  
  type
     EnvException = object of Exception
  
  proc getVar(name: string): string {.inline, raises: [EnvException].} =
     let result = getEnv(name).string
     if len(result) == 0:
        raise newException(EnvException, "%" & name & "% is not defined!")
  
  try:
     let
        yygnu = getVar("YYGNU")
  except EnvException:
     quit(getCurrentExceptionMsg())
  
  let
     cp = yygnu & r"\cp.exe"
  Run

  Please note the "let nature" of yygnu.

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• LeuGim (orginal) [2015-02-13T13:48:37+01:00] view original

  For now you can use try expression for this (as opposed to statement):

  import os
  
  type
     EnvException = object of Exception
  
  proc getVar(name: string): string {.inline, raises: [EnvException].} =
     result = getEnv(name).string
     if len(result) == 0:
        raise newException(EnvException, "%" & name & "% is not defined!")
  
  let
     yygnu = try: getVar("YYGNU") except EnvException: (quit(getCurrentExceptionMsg()); nil)
  
  let
     cp = yygnu & r"\cp.exe"
  Run

  And your defined let result won't be returned from proc, just use the predefined one.

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• AxBen (orginal) [2015-02-13T14:17:27+01:00] view original

  @LeuGim: Thanks for pointing me to the "let result" line (and a practical solution).

  As for "for now", are there any changes in sight?

  As for my example: I hoped to be able to do sth. like this (looks better ;-):

  try:
     let
        v1 = getVar(...)
        v2 = getVar(...)
        v3 = getVar(...)
  except EnvException:
     quit(...)
  
  let
     d1 = f(v1)
     d2 = f(v...)
  Run

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• LeuGim (orginal) [2015-02-13T14:51:16+01:00] view original

  About "for now" - there were suggestions on the forum about settable only once variables, so that you can declare it undefined in the outer block and then assign to it once in some inner block; I don't know is such a feature supposed to appear in future versions or not.

  The problem with your let-variables is that they are defined inside try and try opens new scope (like block; it is intended and quite expected behavior), so v1, ... exist only inside try block. Using var you needed to declare it outside the try and assign to it then inside the try. As let-variables are not assignable, you need to use expression forms of try/if, use helper procs or smth else.

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• AxBen (orginal) [2015-02-13T15:01:47+01:00] view original

  @LeuGim: Thx for the enlightenment! (non ironically)

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• Jehan (orginal) [2015-02-13T15:49:34+01:00] view original

  What you can do in this particular case is something like the following:

  template check(e: expr): expr =
    try: e
    except: quit(getCurrentExceptionMsg()); nil
  
  let
    v1 = check getVar("v1")
    v2 = check getVar("v2")
    v3 = check getVar("v3")
  
  let d1 = f(v1)
  Run

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