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<!DOCTYPE html>
<html>
<head>
<title>Maybe and Either: optional container types</title>
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<h1>SRFI 189: Maybe and Either: optional container types</h1>
<p class="authors">by John Cowan (text) and Wolfgang Corcoran-Mathe (sample implementation)</p>
<p class="based-on"></p>
<p class="status">status: <em>final</em> (2020-07-14)</p>
<p class="keywords">keywords: <a href="https://srfi.schemers.org/?keywords=data-structure">Data Structure</a>, <a href="https://srfi.schemers.org/?keywords=multiple-value-returns">Multiple-Value Returns</a></p><span class="see-also">See also <a href="/srfi-195/">SRFI 195: Multiple-value boxes</a>.</span>
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<li><a href="srfi-189.html">The SRFI Document</a></li>
<li><a href="https://srfi-email.schemers.org/srfi-189/">Discussion Archive</a></li>
<li><a href="https://github.com/scheme-requests-for-implementation/srfi-189">Git repo (on Github)</a></li>
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<h2>Abstract</h2><p>This SRFI defines two disjoint immutable container types
known as Maybe and Either,
both of which can contain objects collectively known as their payload.
A Maybe object is either a Just object or the unique object Nothing
(which has no payload); an Either object is either
a Right object or a Left object. Maybe represents the concept of
optional values; Either represents the concept of values which are
either correct (Right) or errors (Left).</p>
<p>Note that the terms Maybe, Just, Nothing, Either, Right, and Left
are capitalized in this SRFI so as not to be confused with their
ordinary use as English words. Thus "returns Nothing" means
"returns the unique Nothing object"; "returns nothing" could be
interpreted as "returns no values"
or "returns an unspecified value".</p></body></html>