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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Foldable doesn’t have to be a superclass of Traversable.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Removing it only requires a few minor changes to existing code.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Libraries [mailto:libraries-bounces@haskell.org]
<b>On Behalf Of </b>Marcin Mrotek<br>
<b>Sent:</b> 21 February 2016 09:05<br>
<b>To:</b> Jeremy<br>
<b>Cc:</b> Haskell Libraries<br>
<b>Subject:</b> Re: Haskell Foldable Wast<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:4.8pt">It may be that some of the Foldable methods for tuples happen to be useful,<br>
but this does not mean that tuples are foldable.<o:p></o:p></p>
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<p class="MsoNormal">I think that, as far as Foldable is concerned, a tuple is equivalent to Identity, so this instance is indeed useless. However, Foldable is a superclass of Traversable (and it wouldn't make much sense to make these classes unrelated, as
one can always define folds with `traverse`), so I've always found it a necessary evil. Either's instance for Foldable is a little more useful, but equivalent to Maybe's, I think?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">But there are, in general, at least two separate use cases for these instances:<o:p></o:p></p>
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<p class="MsoNormal">a) Using Foldable methods directly on tuples or Eithers - I think it's usually pointless, as I wrote above.<o:p></o:p></p>
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<p class="MsoNormal">b) Passing a tuple/Either/Maybe/whatever to a more complicated function that requires a Foldable constraint. This is potentially much more useful, and at least saves some wrapping/unwrapping.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Best regards,<o:p></o:p></p>
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<p class="MsoNormal">Marcin Mrotek <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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