Experiment with generic NSOperation composition

OperationsComposers.swift

protocol FunctionOperation : class {
  associatedtype InputType
  associatedtype OutputType
  var input:InputType? { get set }
  var output:OutputType? { get set }
  func addDependency(op: NSOperation)
}

/**
 Returns an _adapter block_ which passes the output from `f` into the input of `g`, and builds dependencies so f runs first, then the adapter, then g.
 
 The client is still responsible for adding all three blocks to queues.
 */
func adapterOperationToChainOperations<F:FunctionOperation,G:FunctionOperation
  where F.OutputType == G.InputType>(f:F,g:G) -> NSBlockOperation
{
  guard let fop = f as? NSOperation else { fatalError() }
  guard let gop = g as? NSOperation else { fatalError() }
  
  let adapterOp = NSBlockOperation {
    g.input = f.output
  }
  
  adapterOp.addDependency(fop)
  gop.addDependency(adapterOp)
  
  return adapterOp
}

/**
 Encapsulates a single closure evaluation
 */
class FunctionBlockOperation<InputType,OutputType> : NSOperation, FunctionOperation
{
  let fn:(InputType) -> OutputType
  var input:InputType?
  var output:OutputType?
  
  init(block:(InputType)->OutputType) {
    self.fn = block
  }

  override func main() {
    self.output = self.fn(self.input!)
  }
}