once.swift

/** Wrap a block so that it can be called only once. Calling the returned block twice results in a fatal error. After
the first call but before returning the result from the wrapped block, the wrapped block is dereferenced. */
public func once<P, R>(_ block: ((P) -> (R))) -> ((P) -> (R)) {
	#if DEBUG
		var blockReference: ((P) -> (R))? = block
		let mutex = Mutex()

		return {(p: P) -> (R) in
			let block = mutex.locked { () -> ((P) -> (R)) in
				assert(blockReference != nil, "callback called twice!")
				let r = blockReference!
				blockReference = nil
				return r
			}

			return block(p)
		}
	#else
		return block
	#endif
}

// Use as follows:
func longRunningOperation(_ callback: (String) -> ()) { 
	callback("Hello")
	callback("World") // this will fail!
}

longRunningOperation(once { result in 
	print(result)
})