Question

I am trying to serialize a Contacts type but I am stuck at defining put and get?

import Control.Monad
import Data.Binary

type Name = String
type Address = String

data Contacts = Contacts [(Name, Address)] deriving (Show)
instance Binary Contacts where
    put (Contacts [(n,a)]) = do ...
    get = do ...

main :: IO ()
main = do
    let c = Contacts [("gert","home")]
    let e = encode c
    let d = decode e
    print d

Answer 1

Yes, you are stuck defining put and get. Does that answer your question?

type Name = String
type Address = String

data Contacts = Contacts [(Name, Address)] deriving (Show)
instance Binary Contacts
    put (Contacts [(n,a)]) = do ...
    get = do ...

Since there are already instances:

instance (Binary a) => Binary [a]
instance (Binary a, Binary b) => Binary (a,b)
instance Binary Char

You should just be able to trivially lift the underlying put and get routines:

instance Binary Contacts where
    put (Contacts set) = put set
    get = fmap Contacts get

So when you put contacts you just tell it to put the list of pairs of strings. When you want to deserialize the contacts you just get the underlying list and use the Contacts constructor.