[Haskell-cafe] Newbie Q: Overloading and type classes
Dmitri O.Kondratiev
dokondr at gmail.com
Sat Jun 7 19:07:19 EDT 2008
{--
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 <lrpalmer at gmail.com> wrote:
>
> 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 at gmail.com
http://www.geocities.com/dkondr
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