On Tue, Dec 11, 2012 at 5:54 AM, Mario Blazevic wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">At the instigation of Edward Kmett, I hereby propose to merge<br>




Data.Functor.Coproduct module from comonad-transformers into<br>
transformers.<br>
</blockquote><div><br></div><div>I&#39;ve been wishing this datatype was there.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">An alternative naming for the module (and the type) would be<br>




Data.Functor.Sum.</blockquote><div><br></div><div>I prefer Sum.</div><div><br></div><div>I have a much stronger preference for a new datatype rather than a wrapper around Either. The wrapping/unwrapping doesn&#39;t seem to add anything but more work when you use it (in pattern matching).</div>



<div><br></div><div>Currently, you have:</div><div><br></div><div>&gt; newtype Coproduct f g a = Coproduct { getCoproduct :: Either (f a) (g a) }</div><div><br></div><div>Why not this?</div><div><br></div><div>
&gt; data Sum f g a = SumLeft (f a) | SumRight (g a)</div><div><br></div><div>(I&#39;m not particular about the constructor names, though these do seem to fit the explicit naming style of the transformer functors.) Then, we don&#39;t need the functions left and right, and coproduct becomes:</div>


<div><br></div><div><div>&gt; unSum :: (f a -&gt; b) -&gt; (g a -&gt; b) -&gt; Sum f g a -&gt; b</div><div>&gt; unSum f _ (SumLeft x) = f x</div></div><div>&gt; unSum _ g (SumRight x) = f x</div><div><br></div><div>(Also not particular about the name unSum. Could be runSum or getSum, I suppose.)</div>



<div><br></div><div>Regards,</div><div>Sean</div></div>