I propose adding an (orphan) instance of Monad for ((,) e) to Data.Monoid.<div><div style="color: rgb(34, 34, 34); font-size: 13px; background-color: rgba(255, 255, 255, 0.917969); "><pre style="font-family: arial, sans-serif; white-space: pre-wrap; ">
Discussion deadline: 2 weeks from now (November 26)</pre><pre style="font-family: arial, sans-serif; white-space: pre-wrap; "><span style="font-family: arial, sans-serif; ">== Current State ==</span></pre><div style="font-family: arial, sans-serif; ">
No instance</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">== The Issues ==</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">
Control.Applicative already exports a compatible instance for Applicative for ((,) e), but there isn&#39;t a canonical place to get the compatible Monad.</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">
== Proposed enhancement ==</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">Add the following orphan instance to Data.Monoid.</div><div><pre style="font-family: arial, sans-serif; white-space: pre-wrap; ">
<span style="color: blue; ">instance</span> Monoid e =&gt; Monad <span style="color: red; ">(</span>(,) e<span style="color: red; ">)</span> <span style="color: blue; ">where</span>
<a name="13399687bf91e458_line-24" style="color: rgb(17, 85, 204); "></a>        return a <span style="color: red; ">=</span> <span style="color: red; ">(</span>a, mempty<span style="color: red; ">)</span></pre><pre style="font-family: arial, sans-serif; white-space: pre-wrap; ">
        <span style="color: red; ">(</span>w,a<span style="color: red; ">)</span> &gt;&gt;= k <span style="color: red; ">=</span> <font color="#ff0000">(</font>mappend w w&#39;, b<span style="color: red; ">)</span></pre><pre style="font-family: arial, sans-serif; white-space: pre-wrap; ">
            <span style="color: blue; ">where </span><span style="color: red; ">(</span>w&#39;,b<span style="color: red; ">) = </span>k a</pre><pre style="font-family: arial, sans-serif; white-space: pre-wrap; "><span style="font-family: arial, sans-serif; ">Why?</span></pre>
<pre style="font-family: arial, sans-serif; white-space: pre-wrap; ">Monoid isn&#39;t exported from the Prelude, so there is currently no place for this instance to live currently that doesn&#39;t render it an orphan.</pre>
<div style="font-family: arial, sans-serif; ">== Alternative ==</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">We could place this instance in Control.Monad.Instances which is where we&#39;ve placed similar instances in the past.</div>
<div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">However, this would increase the likelihood of failure to properly import the instance for users, who would have to know to import both Data.Monoid and Control.Monad.Instances, rather than just Data.Monoid.</div>
</div><div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">Moreover in a separate active proposal I&#39;ve just proposed moving all the Control.Monad.Instances instances out, so in the event that that proposal is adopted this would be unsatisfyingly the only instance there.</div>
<div style="font-family: arial, sans-serif; "><br></div><div style="font-family: arial, sans-serif; ">-Edward</div></div></div>