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

[陈文龙]

The Indexable class is declared as

class Lookup f => Indexable f where  index :: f a -> Key f -> a

Why f must be instance of Lookup?


2012/4/12 Michael Sloan <mgsloan at gmail.com>

> 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
> >
>
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