There is a chapter in Real World Haskell [1] devoted to this exact question on this exact piece of code. <br><br>hth,<br>-deech<br><br>[1] <a href="http://book.realworldhaskell.org/read/profiling-and-optimization.html">http://book.realworldhaskell.org/read/profiling-and-optimization.html</a><br>
<br><div class="gmail_quote">On Thu, Apr 23, 2009 at 8:52 AM, Daniel Carrera <span dir="ltr"><<a href="mailto:daniel.carrera@theingots.org">daniel.carrera@theingots.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div class="im">Daniel Fischer wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Try explicitly converting the length to the appropriate type:<br>
<br>
average xs = sum xs / fromIntegral (length xs)<br>
</blockquote>
<br></div>
Thanks. Could you help me understand what's happening?<br>
<br>
1. length returns Int.<br>
2. sum returns Num.<br>
3. (/) wants Fractional.<br>
<br>
It looks like (/) is happy with Num but doesn't like Int. This surprises me. I would have thought that Fractional is a kind of Num and Int is a kind of Fractional, so a function that expects Fractional would be happy with an Int but maybe not with a Num. But clearly that's not the way it works.<br>
<br>
'fromIntegral' converts Int to Num. So obviously, Num is good and Int is bad. But I don't really get why.<div class="im"><br>
<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
will yield a working (albeit inefficient)<br>
average :: Fractional a => [a] -> a<br>
</blockquote>
<br></div>
Why is it inefficient? How would you make it efficient?<br>
<br>
Thanks for the help.<br>
<br>
Cheers,<div><div></div><div class="h5"><br>
Daniel.<br>
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