[jira] [Comment Edited] (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:comment-tabpanel&focusedCommentId=15136387#comment-15136387 ] 

Kevin Klues edited comment on MESOS-4611 at 2/7/16 7:26 PM:
------------------------------------------------------------

No, that has the same problem. [~mpark] summarized for me offline why our current templates can't match this.  The basic problem is that lambda's cant be implicitly cast to the std::function() type, so we end up matching with a different dispatch() template which returns a void. 


was (Author: klueska):
No, that has the same problem.

> 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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