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2375 :: Determining backend configuration at compile time

• xomachine (orginal) [2016-07-13T16:19:58+02:00] view original

  Hello, Nim community!

  I'm working on macros which allows usage of C++ libraries from C backend with minimal effort. During reflection on the idea I've encountered some questions:

  • Is there a way to determine which backend is used to compile current file? It's necessary for constructions like:

  when isCppBackend:
    ... # use typical importcpp pragma
  when isJSBackend:
    ... # error message
  when isCBackend:
    ... # use some magic to represent C++ objects in C terms
  Run

  • Is there a way to obtain Nim or C++ compiler command used at compile time? The idea is to call C++ or Nim (with C++ backend) compiler and run "spy code" to determine class field offsets, mangled names (for fast and unsafe calls) and other C++ specific things.

  The part of the idea realization (without C backend support) can be found here

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• andrea (orginal) [2016-07-13T17:17:40+02:00] view original

  See this example where I give an error on JS backend:

  when defined(js):
    {.fatal: "linalg is only available for native backends".}
  Run

  I assume there should be similar flags for cpp and so on, but I am not sure where to find the exact list (there should be also Objective C and PHP?)

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• xomachine (orginal) [2016-07-13T18:51:40+02:00] view original

  Thank you, andrea!

  It really works! I've made tests and found which keywords work.

  when defined(js):
    echo "Hello JS!"
  when defined(c):
    echo "Hello C!"
  when defined(cpp):
    echo "Hello C++!"
  when defined(objc):
    echo "Hello ObjC!"
  when defined(php):
    echo "Hello PHP!"
  Run

  
  nim c --run test.nim
  nim cpp --run test.nim
  Hello C++!
  nim objc --run test.nim
  Hello ObjC!
  nim js test.nim # Hello JS!
  nim php --run test.nim
  Hello JS!
  Run

  As it can be seen, only cpp, js and objc keywords are correct. JS checker also works with php target.

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• endragor (orginal) [2016-07-14T09:14:09+02:00] view original

  I don't know if such constant is already defined (seems not), but you could use something like this:

  type Backend {.pure.} = enum
    C, CPP, OBJC, JS, PHP
  
  const backend = when defined(cpp): Backend.CPP
                  elif defined(objc): Backend.OBJC
                  elif defined(nimphp): Backend.PHP
                  elif defined(js): Backend.JS
                  else: Backend.C
  Run

  It is a bit tough to manage this as a user, so I think it would make sense to add such constant to system lib.

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