Re: Defaults for superclass methods

[Dave Menendez]

John Meacham writes:

 On Tue, Apr 11, 2006 at 11:35:09AM +0100, Simon Marlow wrote:
  On 11 April 2006 11:08, Ross Paterson wrote:
  
   On Tue, Apr 11, 2006 at 11:03:22AM +0100, Simon Marlow wrote:
   This is a rather useful extension, and as far as I can tell it
   doesn't have a ticket yet: 
   
  
http://www.haskell.org//pipermail/libraries/2005-March/003494.html
   
   should I create a ticket?  Is there any reason it might be hard
   to implement?
   
   There are a range of proposals, but none of them are implemented.
   Wouldn't that rule them out for Haskell'?
  
  If it's not clear which is the right way to go, then yes I guess
  that does rule it out.  Could you summarise the proposals?  If
  there was a clear winner, and it was easy enough to implement,
  perhaps we can knock up a prototype in time.
 
 As I recall, this was brought up a few times during the class alias
 discussion and there were good technical reasons why it would be
 tricky to define a sane semantics for it. as in, it's harder than it
 first looks.

The tricky part is dealing with multiple subclasses.

For example,

class Functor f where
fmap :: (a - b) - f a - f b

class Functor f = Monad f where
...
fmap = liftM

class Functor f = Comonad f where
...
fmap = liftW

newtype Id a = Id a

instance Functor Id
instance Monad Id
instance Comonad Id

Which default gets used for fmap?
-- 
David Menendez [EMAIL PROTECTED] | In this house, we obey the laws
http://www.eyrie.org/~zednenem  |of thermodynamics!
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Defaults for superclass methods

[Simon Marlow]

This is a rather useful extension, and as far as I can tell it doesn't
have a ticket yet:

  http://www.haskell.org//pipermail/libraries/2005-March/003494.html

should I create a ticket?  Is there any reason it might be hard to
implement?

Cheers,
Simon
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Re: Defaults for superclass methods

[Ross Paterson]

On Tue, Apr 11, 2006 at 11:03:22AM +0100, Simon Marlow wrote:
 This is a rather useful extension, and as far as I can tell it doesn't
 have a ticket yet:
 
   http://www.haskell.org//pipermail/libraries/2005-March/003494.html
 
 should I create a ticket?  Is there any reason it might be hard to
 implement?

There are a range of proposals, but none of them are implemented.
Wouldn't that rule them out for Haskell'?

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RE: Defaults for superclass methods

[Simon Marlow]

On 11 April 2006 11:08, Ross Paterson wrote:

 On Tue, Apr 11, 2006 at 11:03:22AM +0100, Simon Marlow wrote:
 This is a rather useful extension, and as far as I can tell it
 doesn't have a ticket yet: 
 
   http://www.haskell.org//pipermail/libraries/2005-March/003494.html
 
 should I create a ticket?  Is there any reason it might be hard to
 implement?
 
 There are a range of proposals, but none of them are implemented.
 Wouldn't that rule them out for Haskell'?

If it's not clear which is the right way to go, then yes I guess that
does rule it out.  Could you summarise the proposals?  If there was a
clear winner, and it was easy enough to implement, perhaps we can knock
up a prototype in time.

The reason being I just hit a case where this would be useful, while
trying to find a nice way to express extensible exceptions.

Cheers,
Simon
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