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 <mgsl...@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 <qzche...@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@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >
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