[Haskell-cafe] Restricted type classes

[Sebastian Fischer]

>> Just because we don't have
>> a use now doesn't mean it might not be useful in the future.

I am suspicious about complicating a design for potential future  
benefits.

However, difference lists provide an example of a type that support  
Pointed more naturally than Applicative: the dlist package [1]  
provides Applicative and Monad instances but only by converting to  
normal lists in between.

Note that even fmap cannot be defined without converting difference  
lists to normal lists in between. The natural interface to difference  
lists would be Pointed (without a Functor superclass) and Monoid.

Sebastian

[1]: http://hackage.haskell.org/package/dlist


-- 
Underestimating the novelty of the future is a time-honored tradition.
(D.G.)