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<title>Conventions - Boost.Outcome documentation</title>
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  <div class="titlepage"><div><div><h1 style="clear: both">Conventions</h1></div></div></div>


<p>You now know everything you need to get started with using Outcome
immediately.</p>

<p>The initial temptation for most beginners will be to use a bespoke
strongly typed enumeration on a case by case basis i.e. a &ldquo;once off&rdquo;
custom <code>E</code> type. This is usually due to experience in other languages
with sum types e.g. Rust, Haskell, Swift etc.</p>

<p>However this is C++! Not Rust, not Swift, not Haskell! I must caution you to always avoid using
custom <code>E</code> types in public APIs. The reason is that every time
library A using custom <code>E1</code> type must interface with library B
using custom <code>E2</code> type, you must map between those <code>E1</code> and <code>E2</code>
types.</p>

<p>This is information lossy, i.e. fidelity of failure gets lost
after multiple translations. It involves writing, and then
<em>maintaining</em>, a lot of annoying boilerplate. It leaks internal
implementation detail, and fails to separate concerns. And one
cannot use <a href="../../reference/macros/tryv.html" class="api-reference"><code>BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr)</code></a>

if there is no convertibility between <code>E</code> types.</p>

<p>The C++ 11 standard library, and Boost,
specifically ships <code>&lt;system_error&gt;</code> for the purpose of wrapping up
individual custom <code>E</code> types into a generic framework, where disparate
custom <code>E</code> types can discover and interact with one another.
That ships with every C++ compiler.</p>

<p>For all these reasons, this is why <code>result</code> and <code>outcome</code> default
the <code>EC</code> type to error code. You should leave that default alone
where possible.</p>

<hr />

<h3 id="tl-dr">tl;dr;</h3>

<p>Please <a href="../../motivation/plug_error_code.html">plug your library into <code>std::error_code</code></a>,
or <a href="../../experimental.html">equivalent</a>, and do not expose
custom <code>E</code> types in ANY public API. <code>result</code> and <code>outcome</code> default
<code>EC</code> to an error code for good reason.</p>


        </div><p><small>Last revised: February 09, 2019 at 15:18:26 UTC</small></p>
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