<div dir="ltr">This change would spontaneously introduce space-leaks in currently non-leaky code.<div><br></div><div>It'd be a debugging nightmare for existing users of the product Monoid instance, of whom there are many, who would just see their code start to throw away all their memory on newer GHC versions and have little or no idea why, if they missed news of this update.</div>
<div><br></div><div>As a result I'm -1 on this proposal.</div><div><br></div><div>That said, some kind of package that provides a well-reasoned Data.Tuple.Lazy data type could see use, as using it would imply consent to those semantics.</div>
<div><br></div><div>I do not object morally to it, like Henning, merely pragmatically.</div><div><br></div><div>-Edward</div><div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sat, Aug 17, 2013 at 4:31 PM, Petr Pudlák <span dir="ltr"><<a href="mailto:petr.mvd@gmail.com" target="_blank">petr.mvd@gmail.com</a>></span> wrote:<br>
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<p style="margin:1.2em 0px!important">Dear haskellers,</p>
<p style="margin:1.2em 0px!important">currently the instances
are defined as</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;overflow:auto;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:nowrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px 3px 3px 3px;display:inline;white-space:pre-wrap;border-radius:3px 3px 3px 3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block;padding:0.5em;color:rgb(51,51,51);background:none repeat scroll 0% 0% rgb(248,248,255)"><span style="color:rgb(68,85,136);font-weight:bold"><span style="color:rgb(51,51,51);font-weight:bold">instance</span> (<span>Monoid</span> a, <span>Monoid</span> b) => <span>Monoid</span> (a,b) <span>where</span></span>
mempty = (mempty, mempty)
(a1,b1) `mappend` (a2,b2) = (a1 `mappend` a2, b1 `mappend` b2)</code></pre>
<p style="margin:1.2em 0px!important">However for some
applications this isn't lazy enough, for example</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;overflow:auto;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:nowrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px 3px 3px 3px;display:inline;white-space:pre-wrap;border-radius:3px 3px 3px 3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block;padding:0.5em;color:rgb(51,51,51);background:none repeat scroll 0% 0% rgb(248,248,255)"><span style="color:rgb(153,153,136);font-style:italic">-- | Build two lists by folding on a pair of `Endo` monoids.</span>
<span style="color:rgb(153,0,0);font-weight:bold">test</span> = head $ appEndo (fst $ foldMap (f &&& f) [<span style="color:rgb(0,153,153)">1.</span>.]) []
<span style="color:rgb(51,51,51);font-weight:bold">where</span>
f = <span>Endo</span> . (:)</code></pre>
<p style="margin:1.2em 0px!important">never terminates because
of the unnecessary pattern matching on the constructor <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:nowrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px 3px 3px 3px;display:inline">(,)</code>
forces evaluation of the whole infinite list.</p>
<p style="margin:1.2em 0px!important">I suggest to change all
Monoid instances for tuples to be like</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;overflow:auto;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:nowrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px 3px 3px 3px;display:inline;white-space:pre-wrap;border-radius:3px 3px 3px 3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block;padding:0.5em;color:rgb(51,51,51);background:none repeat scroll 0% 0% rgb(248,248,255)"> <span style="color:rgb(68,85,136);font-weight:bold"><span style="color:rgb(51,51,51);font-weight:bold">instance</span> (<span>Monoid</span> a, <span>Monoid</span> b) => <span>Monoid</span> (a,b) <span>where</span></span>
mempty = (mempty, mempty)
~(a1,b1) `mappend` ~(a2,b2) = (a1 `mappend` a2, b1 `mappend` b2)
<span style="color:rgb(153,153,136);font-style:italic">-- ^^^ ^^^</span></code></pre>
<p style="margin:1.2em 0px!important">which fixes the problem.</p>
<p style="margin:1.2em 0px!important"> Best regards,<br>
Petr</p>
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