[Haskell-cafe] Newbie Q: Overloading and type classes

[Dmitri O.Kondratiev]

{--

I try to define class Store that will group types implmenting different
storage mechanisms .
All types should support two functions:
1) put (key, value) pair into storage
2) get value from storage corresponding to the given key

As an example I start with the following storage types:
--}

-- simple memory cell
data OneCell k v = Cell(k, v)

-- list of cells
data CellList k v = CLst [(k, v)]

{--
First, without grouping these types into any class, just to illustrate what
these types should do, I define the following functions:
--}
{--
put::(k, v) -> OneCell k v -> OneCell k v
put (k1, v1) (Cell(k, v)) = Cell(k1, v1)

get :: Eq k => k -> OneCell k v -> Maybe v
get k (Cell (_, v)) = Just v

c1 ::OneCell String Int
c1 = Cell("one", 1)

putLst :: (k,v) ->  CellList k v -> CellList k v
putLst (k,v) (CLst xs) = CLst ((k,v) : xs)

getLst :: Eq k => k -> CellList k v -> Maybe v
getLst k (CLst xs) = lookup k xs

cl1 = CLst [("one",1),("two",2),("three",3)]
--}

{--
These work as expected.
Now I try to define Store class that should allow me to overload put & get
functions:
--}

class Store s where
    put :: Eq k => (k, v) -> s  -> s
    get :: k s -> Maybe v
{--
instance Store OneCell where
    put (k1, v1) (Cell(k, v)) = Cell(k1, v1)
    get k (Cell (_, v)) = Just v
--}

{--

I get the following error:

 `OneCell' is not applied to enough type arguments
 Expected kind `*', but `OneCell' has kind `* -> * -> *'
 In the instance declaration for `Store OneCell'
--}

instance Store CellList where
    put (k,v) (CLst xs) = CLst ((k,v) : xs)
    get k (CLst xs) = lookup k xs

{--

I get the following error:

`CellList' is not applied to enough type arguments
Expected kind `*', but `CellList' has kind `* -> * -> *'
In the instance declaration for `Store CellList'
--}

What should I do to create Store class?
Thanks!

-- 
Dmitri O. Kondratiev
dokondr at gmail.com
http://www.geocities.com/dkondr
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