Jan Christiansen <[EMAIL PROTECTED]> wrote in article <[EMAIL PROTECTED]> in
gmane.comp.lang.haskell.cafe:
> The problem is that I don't know how to express the composition of
> type constructors like the function (.) for ordinary functions. I know
> that (map1 . map2) is wrong but it states what I want to express,
> namely the composition of two type constructors. With this I would
> like to define an instance definition like the following.
>
>
> instance (Mapping map1 key1, Mapping map2 key2)
> => Mapping (map1 . map2) (key1, key2) where
> lookup (key1, key2) = lookup key2 . lookup key1
> update (key1, key2) f = update key1 (update key2 f)
You can define the type-level composition yourself. Something like
(untested):
newtype Comp map1 map2 v = Comp (map1 (map2 v))
instance (Mapping map1 key1, Mapping map2 key2)
=> Mapping (Comp map1 map2) (key1, key2) where
lookup (key1, key2) (Comp map) = lookup key2 (lookup key1 map)
update (key1, key2) f (Comp map) = Comp (update key1 (update key2 f) map)
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