[Haskell-cafe] Is type class 'Addressable' already exists.

[Michael Sloan]

Hello!

Yes, classes of that variety exist in a few packages.  This is a
particularly good treatment of it:

http://hackage.haskell.org/package/keys


Here are some classes from a very WIP implementation of a few
"Commutative Replicated Data Types":

https://github.com/mgsloan/crdt/blob/master/src/Data/CRDT/Classes.hs

"Function" is identical to your addressable, without (#).  There're
also classes for "Update"-able, "Zero"-able, and "Size"-able things.
Zero has a strange definition because CRDT sets need to communicate
what has been deleted, clearing a set results in a value that is not
the same as "zero".  I suppose that "clear" aught to be in a separate
class.

-Michael Sloan

On Wed, Apr 11, 2012 at 10:47 PM, 陈文龙 <qzchenwl at gmail.com> wrote:
>
> To get element in List,Map... in python's way.
>
>
>
>
> Python:
>
>
>
>
>> strMap["apple"]
>
>
>
>
> Haskell:
>
>
>
>
>> strMap # "apple"
>
>
>
>
> https://gist.github.com/2364395
>
>
>
>
> {-# LANGUAGE TypeFamilies #-}
>
>
>
>
> module Addressable where
>
> import qualified Data.Map as M
>
> import Prelude
>
>
>
>
> class Addressable a where
>
>     type Key a
>
>     type Value a
>
>     (#!) :: a -> Key a -> Value a
>
>     (#)  :: a -> Key a -> Maybe (Value a)
>
>
>
>
> instance Addressable [a] where
>
>     type Key [a] = Int
>
>     type Value [a] = a
>
>     (#!) = (!!)
>
>     xs     # i | i < 0 = Nothing
>
>     []     # _         = Nothing
>
>     (x:_)  # 0         = Just x
>
>     (_:xs) # n         = xs # (n-1)
>
>
>
>
> instance (Ord k) => Addressable (M.Map k v) where
>
>     type Key (M.Map k v) = k
>
>     type Value (M.Map k v) = v
>
>     a #! i = a M.! i
>
>     a #  i = M.lookup i a
>
>
>
>
> main :: IO ()
>
> main = do
>
>     let strMap = M.fromList [("one","1"),("two","2"),("three","3")]
>
>     let strList = ["1","2","3"]
>
>     print $ strMap  #  "two"      -- Just "2"
>
>     print $ strMap  #! "two"      -- "2"
>
>     print $ strList #  0          -- Just "1"
>
>     print $ strList #! 0          -- "1"
>
>     print $ strMap  #  "no-exist" -- Nothing
>
>     print $ strList #  100        -- Nothing
>
>     print $ strMap  #! "no-exist" -- error
>
>     print $ strList #! 100        -- error
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>