Why are Cofunctor and Comonad classes not a part of the base library? I recently defined a data type (Control.Cofunctor.Ticker in monad-coroutine on Hackage) that happens to be a co-functor, or contra-functor if you prefer. In other words, it can implement the following function:
cofmap :: (a -> b) -> cf b -> cf a
I wanted to define a proper instance for it, but to my surprise I discovered that I couldn't. Not only is the class not defined in base, the only package I could find on Hackage that defines it is category-extras. This is a huge package I'd rather not have as a dependency, so I opted not to declare any instance. Later on I found that this question has been raised before by *Conal Elliott*, nearly four years ago. http://www.haskell.org/pipermail/libraries/2007-January/006740.html The result was the TypeCompose package, which presents a decent solution. I still can't think of any harm in having a proper class declaration in the base library: if you don't need it, you don't need to know it. But without the class declaration in the base library (or in some other obvious package) every other released library must either lack the instance declarations or declare the class itself and risk clashes. This is not some obscure class you've never encountered, by the way: any "consumer" of data is a cofunctor. All regexp data types, for example, are cofunctor instances - or would be if there was a class to declare them instances of. The same question extends to Comonad. I am not too familiar with this area so there may be fewer potential instances of the class, but I remember there was quite a bit of buzz around comonads a few years ago. Judging by a cursory Hayoo search, if anything the situation is worse than with Cofunctor: there are multiple incompatible declarations of the class scattered in various libraries that need it: ad, category-extras, data-category, rope. There is also evidence of undeclared Comonad instances: see the ListZipper and value-supply libraries. So, is it time to add these two classes to the base library?