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--- rfc:sum_types [2015/02/15 00:06] – Fix syntax mistake levim
+++ rfc:sum_types [2015/03/11 15:47] (current) – Deleted levim
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-====== PHP RFC: Sum Types ======
-  * Version: 0.1
-  * Date: 2015-02-14
-  * Author: Levi Morrison <levim@php.net>
-  * Status: Draft
-  * First Published at: http://wiki.php.net/rfc/sum_types
-===== Introduction =====
-In PHP 5.1 the ability to provide explicit types for parameters was added to PHP. It supported classes, interfaces and arrays. In PHP 5.4 the ''callable'' pseudo-type was added. The ability to provide explicit return types was added in PHP 7 but did not allow returning a class or null, a commonly requested feature. Later an RFC for adding explicit scalar types was proposed and a few people wanted to know why there isn't a ''numeric'' type to handle the case where ''int'' and ''float'' are both accepted. I have personally wanted the ability to type against both ''array'' and ''Traversable'' in some of my libraries. All of these requests and discussions suggest that the ability to accept multiple types for a single parameter or return type could be useful. This RFC is an investigation into the feasibility of this idea and to flesh out the impact it would have on other parts of PHP.
-===== Proposal =====
-This RFC proposes the ability to define multiple types for a single parameter or return type and calls them "sum types". Here is an example of a parameter accepting both ''array'' and ''Traversable'' simultaneously:
-<PHP>
-function (array|Traversable $in) {
-    foreach ($in as $value) {
-        echo $value, PHP_EOL;
-    }
-}
-</PHP>
-===== Proposed PHP Version(s) =====
-It is not yet known if this requires backwards compatibility breaks. It could theoretically target PHP 7.NEXT but possibly should wait until PHP 8 (or whatever we call it).
-===== RFC Impact To Opcache =====
-There is not yet an implementation, but this will certainly necessitate changes in opcache.
-===== Open Issues =====
-==== Weak Scalar Types ====
-If PHP adds explicit weak scalar types there is a question of how things will get converted in some situations. As an example, if we have a sum type of ''int'' and ''float'' and are passed the string "10" how is it converted?
-<PHP>
-function f(int|float $number) {
-    return $number * 2;
-}
-f("10");
-</PHP>
-Would it be converted to ''int(10)'' or ''float(10)'', since either is acceptable? Does it matter given they are both acceptable?
-This issue would be avoided if a parameter type exactly matches the input type or if PHP has strict scalar types.
-==== Long Type Names ====
-Since you can create a chain of types the names can get quite lengthy. Even the fairly short sum type of ''array|Traversable'' can be repetitive to type out. Should a method to provide type aliases exist? It may be helpful to look at HHVM's [[http://docs.hhvm.com/manual/en/hack.typealiasing.php|Type Aliasing]].
-==== Reflection ====
-There isn't any part of our current Reflection extension that can deal with sum types and I have not yet given it any thought.
-===== Future Scope =====
-This sections details areas where the feature might be improved in future, but that are not currently proposed in this RFC.
-===== Proposed Voting Choices =====
-This RFC requires that two-thirds of voters vote in favor of the RFC to pass.
-===== Patches and Tests =====
-Presently no patch.
-===== References =====
-Links to external references, discussions or RFCs
-===== Rejected Features =====
-Keep this updated with features that were discussed on the mail lists.

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