[Haskell-cafe] Re: Type variables

2010-03-09 Thread Maciej Piechotka 
On Tue, 2010-03-09 at 15:50 +0100, Giuseppe Maggiore wrote:
 
 class (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
 HasMethod n l a b rec where
 
   (..!) :: l - n - (a - (b,a))
 
  
 
 instance (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
 HasMethod n l a b rec where
 
   l ..! n =
 
  let m = l .! n
 
  in (\x - 
 
   let (y,v) = m x
 
   in (y,convert v))
 
 

i think you have in mind something like:

import Control.Arrow

(..!) :: (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
 l - n - (a - (b,a))
l ..! n = second convert . (l .! n)

1. I don't see a point of creating class. I mean you provide a wildcard
implementation - why not provide just a method?
2. I'm afraid that it might be monomorphism restriction. But I'm not
sure.
3. Without code I can hardly test the problems ;) I can write what I
_think_ code would look like.

Regards
PS. I would be grateful for ASCII-only posts:
http://www.asciiribbon.org/
PPS. convert looks strangly similar to copointed:
http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/Control-Functor-Pointed.html#t%3ACopointed




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Re: [Haskell-cafe] Re: Type variables

2010-03-09 Thread Giuseppe Maggiore 
Thanks, that helped (monomorphism restriction and copointed)...

I'll see if I can come up with a more specific example!


On Tue, Mar 9, 2010 at 6:12 PM, Maciej Piechotka uzytkown...@gmail.comwrote:

 On Tue, 2010-03-09 at 15:50 +0100, Giuseppe Maggiore wrote:
 
  class (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
  HasMethod n l a b rec where
 
(..!) :: l - n - (a - (b,a))
 
 
 
  instance (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
  HasMethod n l a b rec where
 
l ..! n =
 
   let m = l .! n
 
   in (\x -
 
let (y,v) = m x
 
in (y,convert v))
 
 

 i think you have in mind something like:

 import Control.Arrow

 (..!) :: (CNum n, HasField n (a - (b,rec a)) l, Convert rec) =
 l - n - (a - (b,a))
 l ..! n = second convert . (l .! n)

 1. I don't see a point of creating class. I mean you provide a wildcard
 implementation - why not provide just a method?
 2. I'm afraid that it might be monomorphism restriction. But I'm not
 sure.
 3. Without code I can hardly test the problems ;) I can write what I
 _think_ code would look like.

 Regards
 PS. I would be grateful for ASCII-only posts:
 http://www.asciiribbon.org/
 PPS. convert looks strangly similar to copointed:

 http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/Control-Functor-Pointed.html#t%3ACopointed



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-- 
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444
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