Control.Monad, Control.Applicative vs. Data.Foldable, Data.Traversable

[Henning Thielemann]

What is the reason for some of the standard type classes being in 'Data' 
folder and some in 'Control'? Since, say 'Reader' is a monad, an 
applicative thing, an arrow and an instance of other classes - why is it 
located in Control.Monad? Why not Data.Reader? I find it reasonable to 
have a directory for concrete data types and one directory for classes. 
Currently I cannot see, why the Reader data type is in Monad, Monad is in 
Control, but Foldable class is in Data, together with List, Tree and other 
concrete data types.