<div><font color="#3333ff"><font size="4">Thanks, that helped (monomorphism restriction and copointed)...</font></font></div>
<p><font color="#3333ff"><font size="4">I'll see if I can come up with a more specific example!</font></font></p>
<div><br><br></div>
<div class="gmail_quote">On Tue, Mar 9, 2010 at 6:12 PM, Maciej Piechotka <span dir="ltr"><<a href="mailto:uzytkownik2@gmail.com">uzytkownik2@gmail.com</a>></span> wrote:<br>
<blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">
<div class="im">On Tue, 2010-03-09 at 15:50 +0100, Giuseppe Maggiore wrote:<br>><br>> class (CNum n, HasField n (a -> (b,rec a)) l, Convert rec) =><br>> HasMethod n l a b rec where<br>><br>> (..!) :: l -> n -> (a -> (b,a))<br>
><br>><br>><br>> instance (CNum n, HasField n (a -> (b,rec a)) l, Convert rec) =><br>> HasMethod n l a b rec where<br>><br>> l ..! n =<br>><br>> let m = l .! n<br>><br>> in (\x -><br>
><br>> let (y,v) = m x<br>><br>> in (y,convert v))<br>><br>><br><br></div>i think you have in mind something like:<br><br>import Control.Arrow<br><br>(..!) :: (CNum n, HasField n (a -> (b,rec a)) l, Convert rec) =><br>
<div class="im"> l -> n -> (a -> (b,a))<br></div>l ..! n = second convert . (l .! n)<br><br>1. I don't see a point of creating class. I mean you provide a wildcard<br>implementation - why not provide just a method?<br>
2. I'm afraid that it might be monomorphism restriction. But I'm not<br>sure.<br>3. Without code I can hardly test the problems ;) I can write what I<br>_think_ code would look like.<br><br>Regards<br>PS. I would be grateful for ASCII-only posts:<br>
<a href="http://www.asciiribbon.org/" target="_blank">http://www.asciiribbon.org/</a><br>PPS. convert looks strangly similar to copointed:<br><a href="http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/Control-Functor-Pointed.html#t%3ACopointed" target="_blank">http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/Control-Functor-Pointed.html#t%3ACopointed</a><br>
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</div><br><br clear="all"><br>-- <br>Giuseppe Maggiore<br>Ph.D. Student (Languages and Games)<br>Microsoft Student Partner<br>Mobile: +393319040031<br>Office: +390412348444<br><br><br>