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<p class="MsoNormal">Ugh – sent too soon!</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hello – I wanted to add some comments. mean is as you describe.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">mean1 as defined can take a list of any Real type, and return any Fractional type. These types need not be the same, and it’s up to the *<b>caller</b>* of mean1 to say what types they want to give and get returned, e.g. the following is
possible:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">> :m +Data.Ratio<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">> :m +Data.Complex<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">> mean1 [6%5, 2%3] :: Complex Double<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">0.9333333333333333 :+ 0.0<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This may not be what you were expecting? There’s also no need to restrict to Real, since it is valid to calculate the mean of a list of e.g. complex numbers. So maybe you want something like this:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean2 :: (Fractional a) => [a] -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean2 xs = sum xs / genericLength xs<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">(the realToFrac is now unnecessary). The caller still decides the type, but the argument and result type now have to be the same.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">You can’t now do <span style="font-family:"Courier New"">mean2 foo</span>, but you can do
<span style="font-family:"Courier New"">mean2 [1,2,3]</span> (and the 1, 2, 3 are interpreted as the default fractional type, probably Double).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I personally prefer to write “utility” functions to be as generic as possible. (Imagine you’re including them in some standard library for the whole world to use). But I’m sure there are other opinions.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Re performance, there is a comment against “genericLength” to say that it is not as efficient as length. And, as used above, this is the case. So maybe you actually want:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean3 :: (Fractional a) => [a] -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean3 xs = sum xs / fromIntegral (length xs)<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">which is as efficient as mean.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If you are especially interested in performance, you might want to read
<a href="http://book.realworldhaskell.org/read/profiling-and-optimization.html">this</a>. It’s not really “beginner level” but does look at some issues with mean in some detail (and in particular why it can use so much memory and what you can do about that).
Performance of Haskell code is often more difficult to predict than for other languages, for example due to lazy evaluation and some amazing transformations of you code done by GHC.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Incidentally, I am in the middle of drafting a <a href="https://en.wikibooks.org/wiki/User:Davjam2/Numbers">
page about numbers</a> to include in the Haskell wikibook that might be interesting. It discusses types, classes, defaults numeric conversions, etc. I would be very happy if for any feedback (please leave any on the “Discussion” tab on the page).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">FYI: I tested performance (on GHC 8.4.3 on Windows) with this:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">{-<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">compile: ghc -O2 -prof -fprof-auto -rtsopts Mean.hs<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">run: Mean +RTS -p > nul<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">-}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">module Main (main) where<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">import Data.List<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean :: [Double] -> Double<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean xs = sum xs / fromIntegral (length xs)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean1 :: (Real a, Fractional b) => [a] -> b<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean1 xs = realToFrac (sum xs) / genericLength xs<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean2 :: (Fractional a) => [a] -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean2 xs = sum xs / genericLength xs<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean3 :: (Fractional a) => [a] -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean3 xs = sum xs / fromIntegral (length xs)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean4 :: (Fractional a) => [a] -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">mean4 xs = sum xs / fromIntegral (genericLength xs :: Int)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">main :: IO ()<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">main = do<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> let xs = [1 .. 10000000] :: [Double] --may default to Integer unless specified as Double.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> print $ mean xs --change to mean1, etc, for different runs<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">And got:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<td width="83" valign="top" style="width:62.55pt;border:solid windowtext 1.0pt;border-left:none;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">Total time<o:p></o:p></p>
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<td width="144" valign="top" style="width:108.3pt;border:solid windowtext 1.0pt;border-left:none;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">Total alloc<o:p></o:p></p>
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<p class="MsoNormal">mean<o:p></o:p></p>
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<p class="MsoNormal">0.10<o:p></o:p></p>
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<p class="MsoNormal">1,680,096,640 bytes<o:p></o:p></p>
</td>
</tr>
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<p class="MsoNormal">mean1<o:p></o:p></p>
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<p class="MsoNormal">0.17<o:p></o:p></p>
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<p class="MsoNormal">2,560,096,656 bytes<o:p></o:p></p>
</td>
</tr>
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<p class="MsoNormal">mean2<o:p></o:p></p>
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<p class="MsoNormal">0.17<o:p></o:p></p>
</td>
<td width="144" valign="top" style="width:108.3pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">2,560,096,656 bytes<o:p></o:p></p>
</td>
</tr>
<tr>
<td width="63" valign="top" style="width:47.55pt;border:solid windowtext 1.0pt;border-top:none;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">mean3<o:p></o:p></p>
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<td width="83" valign="top" style="width:62.55pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">0.10<o:p></o:p></p>
</td>
<td width="144" valign="top" style="width:108.3pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">1,680,096,640 bytes<o:p></o:p></p>
</td>
</tr>
<tr>
<td width="63" valign="top" style="width:47.55pt;border:solid windowtext 1.0pt;border-top:none;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">mean4<o:p></o:p></p>
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<p class="MsoNormal">0.10<o:p></o:p></p>
</td>
<td width="144" valign="top" style="width:108.3pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoNormal">1,680,096,640 bytes<o:p></o:p></p>
</td>
</tr>
</tbody>
</table>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">mean4 also uses genericLength but is as fast as length. This is due to some cleaverness in GHC, where it uses more efficient code when it knows the required result is integral.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img border="0" width="694" height="2" style="width:7.2291in;height:.0208in" id="Horizontal_x0020_Line_x0020_1" src="cid:image001.png@01D7459A.0824F980"><o:p></o:p></p>
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> Beginners <beginners-bounces@haskell.org> on behalf of Joe King <joeking1809@yahoo.com><br>
<b>Sent:</b> Saturday, May 8, 2021 10:39:50 AM<br>
<b>To:</b> beginners@haskell.org <beginners@haskell.org><br>
<b>Subject:</b> [Haskell-beginners] Function to compute the mean</span> <o:p></o:p></p>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
</div>
<p class="MsoNormal">Greeetings I am new here and pretty new to Haskell.<br>
<br>
I was wondering what are the relative advanatges/disadvatnages of specifying a mean function in these two ways:<br>
<br>
mean :: [Double] -> Double<br>
mean xs = sum xs / fromIntegral (length xs)<br>
<br>
and<br>
<br>
mean1 :: (Real a, Fractional b) => [a] -> b<br>
mean1 xs = realToFrac (sum xs) / genericLength xs<br>
<br>
I understand that mean1 has the advantage that it can be called with lists of any Real type, so would work with things like <br>
<br>
foo :: [Int]<br>
foo = [1,2,3]<br>
<br>
mean foo<br>
-- type mismatch error<br>
<br>
mean1 foo<br>
-- no error<br>
<br>
But suppose that I know I will only ever use lists of Double, is there still any advantage (or disadvantage of using mean1). For example is there any performance benefit by using mean in that case since mean1 has additional function evaluation. <br>
<br>
Are there any other considerations ?<br>
<br>
Thanks in advance<br>
JK<br>
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<p class="MsoNormal"><o:p> </o:p></p>
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