[jira] [Created] (MESOS-785) Extend stout/try to support functional composition

["Ian Downes (JIRA)" <ji...@apache.org>]

Ian Downes created MESOS-785:
--------------------------------

             Summary: Extend stout/try to support functional composition
                 Key: MESOS-785
                 URL: https://issues.apache.org/jira/browse/MESOS-785
             Project: Mesos
          Issue Type: Improvement
            Reporter: Ian Downes
            Priority: Minor


Motivating example was fetching a list of URIs where each 'fetch' is actually a sequence of operations: fetch the uri to a local file, pass the local file on to chmod or extract, and pass on to chown it or the directory. Individual operations needn't be asynchronous.

This can be written using Futures but is potentially confusing when the code is actually synchronous.

Future<Nothing> fetch(
    const CommandInfo& commandInfo,
    const string& directory,
    const HDFS& hdfs,
    const Option<string>& frameworks_home,
    const Option<string>& user)
{
  foreach (const CommandInfo::URI& uri, commandInfo.uris()) {
    bool executable = uri.has_executable() && uri.executable();

    // This code is synchronous!
    Future<string> result =
        _fetch(uri, directory, hdfs, frameworks_home)
          .then(lambda::bind(_chmod, lambda::_1, directory, executable))
          .then(lambda::bind(_extract, lambda::_1, directory, !executable))
          .then(lambda::bind(_chown, lambda::_1, directory, user));
    if (result.isFailed()) {
      LOG(ERROR) << "Fetch of uri '" << uri.value() << "' failed: " << result.failure();
      return Future<Nothing>::failed(result.failure());
    }
  }
  return Nothing();
}

[~bmahler] and I had these thoughts:
* .ifSome() and .ifError() to express control flow.
* .and() for chaining to make it clear the code is synchronous and also that it will short-circuit error.

e.g.
Try<string> result = 
  _fetch(...)
    .and(chmod...)
    .and(extract...)
    .and(chown...);

Thoughts? How do other languages express this?



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