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

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

     [ https://issues.apache.org/jira/browse/MESOS-4611?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Kevin Klues updated MESOS-4611:
-------------------------------
    Description: 
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}

However, lambdas cannot be implicitly cast to a corresponding std::function<R()> type.
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. 


  was:
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. 



> 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
>              Labels: dispatch, libprocess, mesosphere
>
> 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}
> However, lambdas cannot be implicitly cast to a corresponding std::function<R()> type.
> 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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