(Generically) Build Parsers from custom data types?

Question

I\'m working on a network streaming client that needs to talk to the server. The server encodes the responses in bytestrings, for example, \"1\\NULJohn\\NULTeddy\\NUL501\\NU

Answer 1

You can write your own parser - but there is already a package that can do the parsing for you: cassava and while SO is usually not a place to search for library recommendations, I want to include this answer for people looking for a solution, but not having the time to implement this themselves and looking for a solution that works out of the box.

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.Csv
import Data.Vector
import Data.ByteString.Lazy as B
import GHC.Generics

data Person = P { personId :: Int
                , firstName :: String
                , lastName :: String
                } deriving (Eq, Generic, Show)

 -- the following are provided by friendly neighborhood Generic
instance FromRecord Person
instance ToRecord Person

main :: IO ()
main = do B.writeFile "test" "1\NULThomas\NULof Aquin"
          Right thomas <- decodeWith (DecodeOptions 0) NoHeader <$> 
                              B.readFile "test"

          print (thomas :: Vector Person)

Basically cassava allows you to parse all X-separated structures into a Vector, provided you can write down a FromRecord instance (which needs a parseRecord :: Parser … function to work.

Side note on Generic until recently I thought - EVERYTHING - in haskell has a Generic instance, or can derive one. Well this is not the case I wanted to serialize some ThreadId to CSV/JSON and happened to find out unboxed types are not so easily "genericked"!

And before I forget it - when you speak of streaming and server and so on there is cassava-conduit that might be of help.