{--
Thanks!
Yes, you got it right - I "want to make explicit the fact that the type s
depends on k and v.
So I followed your advice and used the most simple way to do what I need:
--}
class Store s where
put :: Eq k => (k, v) -> s k v -> s k v
get :: Eq k => k -> s k v -> Maybe v
instance Store OneCell where
put (k1, v1) (Cell(k, v)) = Cell(k1, v1)
get k (Cell (_, v)) = Just v
instance Store CellList where
put (k,v) (CLst xs) = CLst ((k,v) : xs)
get k (CLst xs) = lookup k xs
storePut :: (Store s, Eq k) => s k v -> k -> v -> s k v
storePut store key value = put (key, value) store
storeGet :: (Store s, Eq k) => k -> s k v -> Maybe v
storeGet key store = get key store
aCell :: OneCell String Int
aCell = Cell("one", 1)
lst :: CellList Int String
lst = CLst [(1, "one"),(2, "two"),(3, "three")]
st1 = storePut aCell "two" 2
st2 = storePut lst 4 "four"
-- v1 = storeGet "one" st2 -- error
v2 = storeGet "one" st1 -- ok
{--
And what does the word "newbie" imply to you when answering my question?
In what case using 'fundeps' and 'associated types' will make sence for this
example?
--}
Thanks again for your great help!
Dima
On Sat, Jun 7, 2008 at 11:11 PM, Luke Palmer
Oops. Should be:
get :: k -> s k v -> Maybe v
And correspondingly for the later examples. After actually using my brain thinking about your problem, and reading the word "Newbie", I would absolutely stay away from the fundeps/associated types business. :-) Try to get this working with Cell and CellList first :-)
Luke
If instead you have cell types which are restricted in what they can store in different ways, you might explore fundeps or associated types:
-- fundeps class Store s k v | s -> k v where put :: (k,v) -> s -> s get :: s -> Maybe v
-- associated types class Store s where type Key s :: * type Value s :: * put :: (Key s, Value s) -> s -> s get :: s -> Maybe (Value s)
But if you can get away with the former, I would recommend that before looking into these advanced extensions.
Luke
-- Dmitri O. Kondratiev dokondr@gmail.com http://www.geocities.com/dkondr