On Thu, May 6, 2010 at 8:55 AM, Pavel Perikov
Hi, list!.
Now in 6.12.1 we have DeriveFunctor, DeriveFoldable and DeriveTraversable. This greatly simplifies the "reuse structure" style of programming. Some structure (not just _data_ structure) got captured in ADT and can be reused for various purposes.
Wouldn't it be nice to have the ability to derive Applicative as well? It shouldn't be more difficult than deriving Functor but will provide exciting possibilities. Just think about liftA2.
The difference is that there is at most one law-abiding instance of Functor for each type, whereas there are in principle multiple possible instances for Applicative for a type. E.g. the following: instance Applicative [] where pure x = [x] fs <*> xs = concatMap (\f -> map f xs) fs instance Applicative [] where pure = repeat (f:fs) <*> (x:xs) = f x : fs <*> xs _ <*> _ = [] are both law-abiding instances (although only one has a corresponding law-abiding Monad, I believe). Which should GHC choose? It's worth noting, though, that there are other derivable classes that don't have a single implementation. It's a question of trading off complexity of the compiler versus saved effort in code versus additional clarity in code, I think.