[Haskell-cafe] Restricted type classes

Sebastian Fischer sebf at informatik.uni-kiel.de
Sun Sep 5 10:11:44 EDT 2010

>> 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  

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.


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

Underestimating the novelty of the future is a time-honored tradition.

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