Am 01/04/2016 um 03:53 PM schrieb Imants Cekusins:
is it impossible to write a functor instance for ordered lists?
is such specialized functor instance necessary?
I mean, why not fmap over unconstrained list and init OrdList before passing it to a fun where sorted list is really essential?
this would eliminate the need to maintain sorted order at every step in list processing.
This way, OrdList type would ensure that sort-critical consumer fun gets a sorted list, and every other fun - fmap or not - would not care.
I see. The reason why I am asking is I tried to model a predicate on nested items and I came up with this: data Product a = Prod (a -> Maybe (Product a)) | Pany The idea was that given an Item a, a Product would return Nothing if the toplevel Item (the "container") does not statisfy the predicate. Otherwise it returns Just a new Product which captures the condition which each of the contained items must satisfy. That alone did not buy me much, particularly becuase I needed a way to "show" a product. So I needed a showable data structure, which can be used like a Product, i.e. which can be converted into a Product. I came up with data MP a = MPacked (M.Map a (MP a)) | MPany deriving (Show) Then I tried to define a Functor instance and failed for the afforementioned reasons: a Map needs Ord keys. I found this puzzling because a Functor on Product makes perfect sense to me. I assume, this is because M.Map is implemented in such a way, that Ord a is required. Had I used a List of Pairs instead of a Map, then no Ord constraint would have been required. Does this make some sense?