As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance.
data Record a b = R { field1 :: a, field2 :: b } If I want to use fmap to transform either field, I have to declare the type to have the corresponding type variable at the end, i.e. choosing "Record a b" or "Record b a" is already a design decision, and it is driven by the standard Functor class in this case. I can define custom functions fmap1 and fmap2 manually, but then I don't get the advantages of overloading, like fmapping over a data structure containing my records. Now I understand that I can't get everything, and my question is mainly what to do when such a dilemma comes up. Those who have already encountered such a dilemma: how did it come up and what did you do to solve it? Gergely -- http://www.fastmail.fm - Same, same, but different...