Ok, then I will carefully complete and refactor my new combinators (and rewrite part of the manual ;-}{) and come up with a proposal, Doaitse On 8 okt 2008, at 14:20, Ross Paterson wrote:
On Wed, Oct 08, 2008 at 12:36:02PM +0200, S. Doaitse Swierstra wrote:
Stimulated by remarks made during the discussion on the future of Haskell at the last Haskell Symposium, I have started to convert my new parsing library (constructed for the Lernet summerschool in Uruguay) into Cabalised form. In this library I have amongst others the class:
class Applicative p where (<*>) :: p (b -> a) -> p b -> p a (<|>) :: p a -> p a -> p a (<$>) :: (b -> a) -> p b -> p a pReturn :: a -> p a pFail :: p a f <$> p = pReturn f <*> p
which extends/deviates from the standard class Applicative, since I think these functions more or less belong together. I am happy to factor out <|> into a separate class.
This corresponds to Alternative, a subclass of Applicative (except for <$> being a function instead of a method). They certainly belong together for parsers, but there are applicative functors that don't have the extra monoidal structure.
The problem which arises now is when I want to use the class Applicative as it is now defined in Control.Applicative. Functions like <$>, < $, <* and many have standard implementations in terms of the basic function pure and <*>. Although this looks fine at first sight, this is not so fine if we want to give more specialised (optimised, checking) implementations, as I am doing in my library. An example of this is e.g. in many, where I want to check that the parameter parser does not recognise the empty sequence since thi is non-sense, etc. Of course we can describe <* by
p <* q = pure const <*> p <*> q
but this is also rather inefficient; why first building a result if you are going to throw it away anyway?
The current definition isn't quite as bad as that:
p <* q = const <$> p <*> q f <$> a = fmap f a
but the general point stands.
More in general I think it is preferred to place common patterns in classes with a default implementation, so they can be redefined instead of defining them at top level.
1) Does everyone agree with this observation, and if not what am I missing? 2) Can we change the module Applicative to reflect my view? I think it can be done without implying heavy changes to other modules.
This seems reasonable to me, as long as there aren't too many of them; we do this for lots of other classes. Would you like to list all the functions that you would like to have as redefinable methods? _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries