Hi, On 15.09.2010, at 11:40, José Pedro Magalhães wrote:
I'm not sure I fully understand what you're trying to do. Could you provide typical use cases of your function, to several arguments of different types?
Consider a polymorphic data type with one type variable like lists. I want to replace every value of the "polymorphic" type by a list of Booleans that corresponds to its position in the term (with respect to the :*: constructor). For example, shape [1,2] = [[False],[True,False]] because the generic representation (without the injections) of [1,2] is K 1 :*: (K 2 :*: U)) and in this term the path [False] corresponds to 1 while the path [True,False] corresponds to 2. As another example consider the following data type for binary trees. data Tree a = Node (Tree a) a (Tree a) | Leaf a then we have shape (Node (Leaf 'a') 'b' (Leaf 'c')) = Node (Leaf [False]) [True,False] (Leaf [True,True]) because the generic representation (without the injections) of the argument tree is K 'a' :*: (K 'b' :*: K 'c'). In fact, for the same reason I suspect that it is not possible to implement shape by means of regular I would expect that it is not possible to implement the standard fmap by means of regular. Is this correct? Cheers, Jan