[jira] [Created] (MESOS-4611) Passing a lambda to dispatch() always matches the template returning void

["Kevin Klues (JIRA)" <ji...@apache.org>]

Kevin Klues created MESOS-4611:
----------------------------------

             Summary: Passing a lambda to dispatch() always matches the template returning void
                 Key: MESOS-4611
                 URL: https://issues.apache.org/jira/browse/MESOS-4611
             Project: Mesos
          Issue Type: Bug
          Components: libprocess
            Reporter: Kevin Klues


The following idiom does not currently compile:

{code}
  Future<Nothing> initialized = dispatch(pid, [] () -> Nothing {
    return Nothing();
  })
{code}

This seems non-intuitive because the following template exists for dispatch:

{code}
template <typename R>
Future<R> dispatch(const UPID& pid, const std::function<R()>& f)
{
  std::shared_ptr<Promise<R>> promise(new Promise<R>()); 
 
  std::shared_ptr<std::function<void(ProcessBase*)>> f_(
      new std::function<void(ProcessBase*)>(
          [=](ProcessBase*) {
            promise->set(f());
          }));

  internal::dispatch(pid, f_);
  
  return promise->future();
}     
{code}

To make this work, you have to explicitly type the lambda before passing it to dispatch.

{code}
  std::function<Nothing()> f = []() { return Nothing(); };
  Future<Nothing> initialized = dispatch(pid, f);
{code}

We should add template support to allow lambdas to be passed to dispatch() without explicit typing. 




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