Editing Andrei Alexandrescu (section)

=== Expected ===

'''Expected''' is a [[template (C++)|template]] class for [[C++]] which is on the C++ Standards track.<ref>[{{cite web|author1=Botet|author2=Talbot|title=A proposal to add a utility class to represent expected monad|url=http://www.hyc.io/boost/expected-proposal.pdf|url-status=dead|archive-url=https://web.archive.org/web/20140819082937/http://www.hyc.io/boost/expected-proposal.pdf|archive-date=19 August 2014}}</ref><ref>{{Cite web|url=https://github.com/viboes/std-make/blob/master/doc/proposal/expected/p0323r2.md#motivation|title=STD-make/P0323r2.md at master · viboes/STD-make|website=[[GitHub]]|date=21 October 2021}}</ref> Alexandrescu proposes<ref>{{cite web|last1=Alexandrescu|title=Systematic Error Handling in C++|url=http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Andrei-Alexandrescu-Systematic-Error-Handling-in-C|url-status=dead|archive-url=https://web.archive.org/web/20130425035429/http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Andrei-Alexandrescu-Systematic-Error-Handling-in-C|archive-date=25 April 2013}}</ref> <code>Expected<T></code> as a class for use as a return value which contains either a T or the exception preventing its creation, which is an improvement over use of either return codes or exceptions exclusively.  Expected can be thought of as a restriction of sum (union) types or algebraic datatypes in various languages, e.g., [[Hope (programming language)|Hope]], or the more recent [[Haskell (programming language)|Haskell]] and [[Coq (software)|Gallina]]; or of the error handling mechanism of Google's [[Go (programming language)|Go]], or the [https://doc.rust-lang.org/std/result/ Result] type in [[Rust (programming language)|Rust]].

He explains the benefits of <code>Expected<T></code> as:
* Associates errors with computational goals
* Naturally allows multiple exceptions in flight
* Switch between "error handling" and "exception throwing" styles
* Teleportation possible across thread boundaries, across nothrow subsystem boundaries and across time (save now, throw later)
* Collect, group, combine exceptions

==== Example ====

For example, instead of any of the following common function prototypes:

<code>
int parseInt(const string&);  // Returns 0 on error and sets errno.
</code>

or

<code>
int parseInt(const string&);  // Throws invalid_input or overflow
</code>

he proposes the following:

<code>
Expected<int> parseInt(const string&);  // Returns an expected int: either an int or an exception
</code>