Source   Edit  

Once for Nim.

Example:

import threading/once

type
  Singleton = object
    data: int

var
  counter = 1
  instance: ptr Singleton
  exceptionOccurred = false
  o = createOnce()

proc getInstance(): ptr Singleton =
  once(o):
    if not exceptionOccurred:
      # Simulate an exception on the first call
      exceptionOccurred = true
      raise newException(ValueError, "Simulated error")
    instance = createSharedU(Singleton)
    instance.data = counter
    inc counter
  result = instance

proc worker {.thread.} =
  try:
    for i in 1..1000:
      let inst = getInstance()
      assert inst.data == 1
  except ValueError:
    echo "Caught expected ValueError"

var threads: array[10, Thread[void]]
for i in 0..<10:
  createThread(threads[i], worker)
joinThreads(threads)
deallocShared(instance)
echo "All threads completed"