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0253-Document-opaques.patch
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File 0253-Document-opaques.patch of Package erlang
From 3d75a9928a3c27b06eb313f926dee92c2a57a8d8 Mon Sep 17 00:00:00 2001 From: Maxwell Elliot Heiber <mheiber@fb.com> Date: Sat, 18 Dec 2021 18:35:15 +0000 Subject: [PATCH 1/3] Document opaques close #5240 --- system/doc/reference_manual/opaques.xml | 69 ++++++++++++++++++++++++ system/doc/reference_manual/typespec.xml | 1 + 2 files changed, 70 insertions(+) create mode 100644 system/doc/reference_manual/opaques.xml diff --git a/system/doc/reference_manual/opaques.xml b/system/doc/reference_manual/opaques.xml new file mode 100644 index 0000000000..1514ab01cb --- /dev/null +++ b/system/doc/reference_manual/opaques.xml @@ -0,0 +1,69 @@ +<?xml version="1.0" encoding="utf-8" ?> +<!DOCTYPE chapter SYSTEM "chapter.dtd"> + +<chapter> + <header> + <copyright> + <year>2021</year><year>2022</year> + <holder>Ericsson AB. All Rights Reserved.</holder> + </copyright> + <legalnotice> + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. + + </legalnotice> + + <title>Types and Function Specifications</title> + <prepared>Max Heiber</prepared> + <docno></docno> + <date></date> + <rev></rev> + <file>opaques.xml</file> + </header> + + <section> + <title>Opaque Type Aliases</title> + <p>The main use case for opacity in Erlang is to hide the implementation of a data type, enabling evolving the API while minimizing risk of breaking consumers. The runtime does not check opacity. Dialyzer provides some opacit-checking, but the rest is up to convention. + </p> + <p> +This document explains what Erlang opacity is (and the trade-offs involved) via the example of OTP's `sets:set()` data type. This type *was* defined in `sets` module like this: + </p> + + <pre>-opaque set(Element) :: #set{segs :: segs(Element)}.</pre> + OTP 24 changed the definition to the following, in <a href="https://github.com/erlang/otp/commit/e66941e8d7c47b973dff94c0308ea85a6be1958e">this commit</a>. + <pre>-opaque set(Element) :: #set{segs :: segs(Element)} | #{Element => ?VALUE}.</pre> +<p>And this change was safer and more backwards-compatible than if the type had been defined with <pre>-type</pre> instead of <pre>-opaque</pre>. Here's why: when a module defines an <pre>-opaque</pre>, the contract is that only the defining module should rely on the definition of the type: no other modules should rely on the definition.</p> +<p>This means that code that pattern-matched on <pre>set</pre> as a record/tuple technically broke the contract, and opted in to being potentially broken when the definition of <pre>set()</pre> changed. Before OTP 24, this code printed <pre>ok</pre>. In OTP 24 it may error:</p> + <pre> + Set = sets:new(), +caseof + X when is_tuple(Set) -> + io:format("ok") +end. + </pre> +<p><strong>When working with an opaque defined in another module, here are some recommendations:</strong></p> + +<list type="bulleted"> +<item>Don't examine the underlying type using pattern-matching, guards, or functions that reveal the type, such as <pre>tuple_size/1</pre>.</item> +<item>Instead, use functions provided by the module for working with the type. For example, <pre>sets</pre> module provides <pre>sets:new/0</pre>, <pre>sets:add/2</pre>, <pre>sets:is_element/2</pre>, etc.</item> +<item><pre>sets:set(a)</pre> is a subtype of <pre>sets:set(a | b)</pre> and not the other way around. Generally, you can rely on the property that <pre>the_opaque(T)</pre> is a subtype of <pre>the_opaque(U)</pre> when T is a subtype of U.</item> +</list> +<p><strong>When defining your own opaques, here are some recommendations:</strong></p> +<list type="bulleted"> +<item>Since consumers are expected to not rely on the definition of the opaque type, you must provide functions for constructing and querying/deconstructing intances of your opaque type. For example, sets can be constructed with <pre>sets:new/0</pre>, <pre>sets:from_list/1</pre>, <pre>sets:add/2</pre>, queried with <pre>sets:is_element/2</pre>, and deconstructed with <pre>sets:to_list/1</pre>.</item> +<item>Don't define an opaque with a type variable in parameter position. This breaks the normal and expected behavior that (for example) <pre>my_type(a)</pre> is a subtype of <pre>my_type(a | b)</pre></item> +</list> +<p>Note that opaques can be harder to work with for consumers, since the consumer is expected not to pattern-match and must instead use functions that the author of the opaque type provides to use instances of the type.</p> +<p>Also, opacity in Erlang is skin-deep: the runtime does not enforce opacity-checking. So now that sets are implemented in terms of maps, an <pre>is_map</pre> check on a set <em>will</em> pass. The opacity rules are only enforced by convention and by additional tooling such as Dialyzer. And this enforcement is not total: For example, determined consumer of <pre>sets</pre> can still do things that reveal the structure of the set, such as by printing, serializing, or using a set as <pre>term()</pre> and then inspecting via functions like <pre>is_map</pre> or <pre>maps:get/2</pre>. And Dialyzer must make some <a href="https://github.com/erlang/otp/issues/5118">approximations</a>. Opacity checking has limitations, but is still a vital tool in scalable Erlang development.</p> + + </section> +</chapter> diff --git a/system/doc/reference_manual/typespec.xml b/system/doc/reference_manual/typespec.xml index a450f85742..c80d4f639d 100644 --- a/system/doc/reference_manual/typespec.xml +++ b/system/doc/reference_manual/typespec.xml @@ -405,6 +405,7 @@ types are not accessible by other modules anyway - and is always to be exported. </p> + <p>Read more on <seeguide marker="opaques">Opaques</seeguide></p> </section> <section> -- 2.31.1
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