add IsString instance for Document and update Readme

Author: Marco Zocca

README.md

@@ -33,22 +33,24 @@ VP-trees are a space-partitioning data structure that significantly accelerates
 ```haskell
 {-# LANGUAGE OverloadedStrings #-}
 
-import qualified Data.ByteString.Lazy.Char8 as BL
-import Data.NCDTree
+from Data.NCDTree import Document, mkVPTree, knnSearch
 
--- Create documents from bytestrings
-let docs = [ Document (BL.pack "hello world")
-           , Document (BL.pack "hello universe")
-           , Document (BL.pack "goodbye world")
-           , Document (BL.pack "hello there")
-           ]
+-- Create documents using string literals (thanks to IsString instance)
+docs :: [Document]
+docs = [ "hello world"
+        , "hello universe"
+        , "goodbye world"
+        , "hello there"
+        ]
 
 -- Build the VP-tree index with a leaf threshold of 4
-let tree = mkVPTree 4 docs
+tree = mkVPTree 4 docs
 
 -- Search for the 2 nearest neighbors of "hello"
-let query = Document (BL.pack "hello")
-let results = knnSearch 2 query tree
+query :: Document
+query = "hello"
+
+results = knnSearch 2 query tree
 
 -- results is a max-heap with the closest documents
 ```

src/Data/NCDTree.hs

@@ -10,9 +10,11 @@ module Data.NCDTree
   ) where
 
 import Control.Monad.ST
+import Data.String (IsString(..))
 import Data.Ord (Down(..), comparing)
 
 import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString.Lazy.Char8 as BLC
 
 import qualified Data.Heap as H
 
@@ -26,6 +28,9 @@ newtype Document = Document
     { docText :: BL.ByteString
     } deriving (Eq, Show)
 
+instance IsString Document where
+  fromString = Document . BLC.pack
+
 data VPTree = VPNode { pivot    :: Document
                      , threshold :: Double
                      , inside   :: VPTree