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<p>Hi,</p>
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</p>
<p>I have been recently playing around with GADTs, using them to keep track of how many times a functor has been applied. This provides a solution to what seems to be a long standing problem, summarized here:</p>
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<p><a title="Ctrl+Click or tap to follow the link" id="LPlnk383182" href="https://byorgey.wordpress.com/2007/08/16/mapping-over-a-nested-functor/">https://byorgey.wordpress.com/2007/08/16/mapping-over-a-nested-functor/</a></p>
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<p>If the nesting depth is part of the type, it is possible to apply fmap automatically as needed. This allows you to write fairly elegant expressions like:</p>
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<p><span class="pl-k">data</span> <span class="pl-c1">Person</span> <span class="pl-k">
=</span> <span class="pl-c1">Person</span> { name <span class="pl-k">::</span> <span class="pl-c1">
String</span>, age <span class="pl-k">::</span> <span class="pl-c1">Integer</span>, gender
<span class="pl-k">::</span> <span class="pl-c1">String</span>, status <span class="pl-k">
::</span> <span class="pl-c1">String</span> } <span class="pl-k">deriving</span> <span class="pl-c1">
Show</span> <br>
</p>
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<p>let persons <span class="pl-k">=</span> fromList' [<span class="pl-c1">Person</span> {name<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Alice<span class="pl-pds">"</span></span>, age<span class="pl-k">=</span><span class="pl-c1">20</span>,
gender<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>F<span class="pl-pds">"</span></span>, status<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Good<span class="pl-pds">"</span></span>},
<span class="pl-c1">Person</span> {name<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Bob<span class="pl-pds">"</span></span>, age<span class="pl-k">=</span><span class="pl-c1">18</span>, gender<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>M<span class="pl-pds">"</span></span>,
status<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Good<span class="pl-pds">"</span></span>},
<span class="pl-c1">Person</span> {name<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Chuck<span class="pl-pds">"</span></span>, age<span class="pl-k">=</span><span class="pl-c1">16</span>, gender<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>M<span class="pl-pds">"</span></span>,
status<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>Bad<span class="pl-pds">"</span></span>}]
<span class="pl-k">::</span> <span class="pl-c1">NList</span> N1 <span class="pl-c1">
Person</span><br>
</p>
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<p>persons `select` age<br>
</p>
<p>-- [20,18,16]</p>
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<p>persons `groupby` gender `select` age</p>
<p>-- [[20],[18,16]]</p>
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<p>persons `groupby` gender `groupby` status `select` age</p>
<p>-- [[[20]],[[18],[16]]]<br>
</p>
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<p>Note that "`select` age" works regardless of nesting depth. You can also append or remove nesting levels:<br>
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<p></p>
<p>persons `groupby` gender `select` age `produce` (\x -> [0..x])</p>
-- [[[0..20]],[[0..18],[0..16]]]
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<p>persons `groupby` gender `reduce` (sumof age)</p>
<p>-- [20, 34]<br>
</p>
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<p>Would this kind of thing be of interest? The code is here:</p>
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<p><a title="Ctrl+Click or tap to follow the link" id="LPlnk826216" href="https://github.com/filonik/nlists">https://github.com/filonik/nlists</a></p>
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<p>Please feel free to suggest improvements.</p>
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<p>Cheers,</p>
<p>Daniel<br>
</p>
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