<div>Hi Igor,</div><div><br></div>You can create a new type for a -> Bool with which you can compose to create complex filters. I find that an elegant approach:<div><br></div><div><div>import Data.Monoid </div><div><br>
</div><div>newtype Filter a = Filter {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>runFilter :: a -> Bool </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div>
-- We can turn filter into a monoid, behaving like and. </div><div>instance Monoid (Filter a) where </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>mempty = Filter $ const True </div><div><span class="Apple-tab-span" style="white-space:pre"> </span>mappend (Filter a) (Filter b) = Filter $ \x -> a x && b x </div>
<div><br></div><div><br></div><div>andF :: Filter a -> Filter a -> Filter a </div><div>andF = mappend </div><div><br></div><div>-- We can also create or</div><div>orF :: Filter a -> Filter a -> Filter a</div>
<div>
orF (Filter a) (Filter b) = Filter $ \x -> a x || b x</div><div><br></div><div>mkFilter :: (a -> Bool) -> Filter a </div><div>mkFilter = Filter</div><div><br></div><div>-- Some example filters </div><div>evenFilter = mkFilter even </div>
<div>lessThanHundred = mkFilter (<100) </div><div><br></div><div>specificFilter :: Filter a -> [a] -> [a]</div><div>specificFilter xs ys = filter (runFilter xs) ys</div><div><br></div><div><br></div><div>-- get less then hundred and even</div>
<div>testa = specificFilter (evenFilter <> lessThanHundred) [1..1000]</div><div><br></div><div>-- get less then hundred or even </div><div>testb = specificFilter (evenFilter `orF` lessThanHundred) [1..1000]</div><div>
<br></div><div>Cheers,</div><div><br></div><div>Edgar</div><div><br><div class="gmail_quote">On Sun, Jul 7, 2013 at 7:26 PM, Igor Pinheiro Leão <span dir="ltr"><<a href="mailto:ivpcl@cin.ufpe.br" target="_blank">ivpcl@cin.ufpe.br</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div>Hi Guys,<br>sorry in upsetting you again.<br>Is there a way in which I can filter one list of function, one function at time, to a list of elements, declaring this and only this function.<br>
<br>It would be exactly like this:<br>
<br>specifcFilter :: [(a->Bool)] -> [a] -> [a]<br><br>where for each element of type 'a' on the second list it would exist a function on the first list that would be applied to it filtering. <br><br></div>
Kind regards,<br></div>Igor<span class="HOEnZb"><font color="#888888"><br><div><div><div><div><br clear="all"><div><div><br>-- <br>Igor Vinícius<br>Graduando em Ciência da Computação<br>
</div></div></div></div></div></div></font></span></div>
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