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