On Wednesday 25 May 2005 17:20, Chung-chieh Shan wrote:
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)
Thanks. That's a solution. There is no prettier way of doing this, is there.
It's not allowed to make an instance of a type synonym and partially
applied type synonyms are not allowed anyway. Is there a reason why
partial application of type synonyms are forbidden?
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