haskell safecopy example - haskell

Question

(src http://hackage.haskell.org/packages/archive/safecopy/0.6.1/doc/html/Data-SafeCopy.html)
If you rename Contacts data type into data Contacts_v0
type Name = String
type Address = String
data Contacts = Contacts [(Name, Address)]
instance SafeCopy Contacts where
putCopy (Contacts list) = contain $ safePut list
getCopy = contain $ Contacts <$> safeGet
How is Contacts_v0 suppose to be assigned to old existing data?
type Name = String
type Address = String
type Phone = String
data Contacts_v0 = Contacts_v0 [(Name, Address)]
instance SafeCopy Contacts_v0 where
putCopy (Contacts_v0 list) = contain $ safePut list
getCopy = contain $ Contacts_v0 <$> safeGet
data Contact = Contact { name :: Name
, address :: Address
, phone :: Phone }
instance SafeCopy Contact where
putCopy Contact{..} = contain $ do safePut name; safePut address; safePut phone
getCopy = contain $ Contact <$> safeGet <*> safeGet <*> safeGet
data Contacts = Contacts [Contact]
instance SafeCopy Contacts where
version = 2
kind = extension
putCopy (Contacts contacts) = contain $ safePut contacts
getCopy = contain $ Contacts <$> safeGet
instance Migrate Contacts where
type MigrateFrom Contacts = Contacts_v0
migrate (Contacts_v0 contacts) = Contacts [ Contact{ name = name
, address = address
, phone = "" }
| (name, address) <- contacts ]
From the above library documentation I am trying to do this.
{-# LANGUAGE RecordWildCards, TypeFamilies #-}
import Control.Applicative
import Data.SafeCopy
type Name = String
type Address = String
type Phone = String
data Contacts = Contacts [(Name, Address)] deriving (Show)
instance SafeCopy Contacts where
putCopy (Contacts list) = contain $ safePut list
getCopy = contain $ Contacts <$> safeGet
data Contacts_v0 = Contacts_v0 [(Name, Address)] deriving (Show)
instance SafeCopy Contacts_v0 where
putCopy (Contacts_v0 list) = contain $ safePut list
getCopy = contain $ Contacts_v0 <$> safeGet
data Contact = Contact { name :: Name, address :: Address, phone :: Phone } deriving (Show)
instance SafeCopy Contact where
putCopy Contact{..} = contain $ do safePut name; safePut address; safePut phone
getCopy = contain $ Contact <$> safeGet <*> safeGet <*> safeGet
{-
data Contacts = Contacts [Contact]
instance SafeCopy Contacts where
version = 2
kind = extension
putCopy (Contacts contacts) = contain $ safePut contacts
getCopy = contain $ Contacts <$> safeGet
instance Migrate Contacts where
type MigrateFrom Contacts = Contacts_v0
migrate (Contacts_v0 contacts) = Contacts [ Contact{ name = name, address = address, phone = "" }
| (name, address) <- contacts ]
-}
main :: IO ()
main = do
let test = Contacts [("gert","home")]
print test
--let testNew = how do you migrate test using migrate?
--print testNew
Note that it would make more sense to me if they renamed the new one to Contacts_v2 instead of renaming the old one.
Maybe I should rephrase the question, when is safecopy useful?

Answer 1

{-# LANGUAGE RecordWildCards, TypeFamilies#-}
import Control.Applicative
import Data.SafeCopy
import Data.Binary
import Data.Serialize.Get
import Data.Serialize.Put
type Name = String
type Address = String
type Phone = String
data Contact = Contact { name :: Name, address :: Address, phone :: Phone } deriving (Show)
instance Binary Contact where
put Contact{..} = do put name; put address; put phone
get = do name <- get; address <- get; phone <- get; return Contact{..}
instance SafeCopy Contact where
putCopy Contact{..} = contain $ do safePut name; safePut address; safePut phone
getCopy = contain $ Contact <$> safeGet <*> safeGet <*> safeGet
data Contacts = Contacts [Contact] deriving (Show)
instance Binary Contacts where
put (Contacts set) = put set
get = fmap Contacts get
instance SafeCopy Contacts where
version = 2
kind = extension
putCopy (Contacts contacts) = contain $ safePut contacts
getCopy = contain $ Contacts <$> safeGet
instance Migrate Contacts where
type MigrateFrom Contacts = Contacts_v0
migrate (Contacts_v0 contacts) = Contacts[Contact{name=name,address=address,phone=""}|(name,address)<-contacts]
data Contacts_v0 = Contacts_v0 [(Name, Address)] deriving (Show)
instance Binary Contacts_v0 where
put (Contacts_v0 set) = put set
get = fmap Contacts_v0 get
instance SafeCopy Contacts_v0 where
putCopy (Contacts_v0 list) = contain $ safePut list
getCopy = contain $ Contacts_v0 <$> safeGet
main :: IO ()
main = do
--
-- instance Binary
--
let c' = Contacts[Contact{name="gert",address="home",phone="test"},Contact{name="gert2",address="home2",phone="test2"}]
let e' = encode c'
print e'
let d' = decode e'
print (d':: Contacts)
let c = Contacts_v0 [("gert_v0","home_v0"),("gert2_v0","home2_v0")]
let e = encode c
print e
let d = decode e
print (d:: Contacts_v0)
--can not do print (d:: Contacts) meaning you are screwed
--
-- instance SafeCopy
--
let c'' = Contacts_v0 [("gert_v0","home_v0"),("gert2_v0","home2_v0")]
let e'' = runPut (safePut c'')
print e''
let d'' = runGet safeGet e''
case d'' of
Left _ -> print "error"
Right d'' -> print (d'':: Contacts)
--can do print (d:: Contacts) or print (d:: Contacts_v0) meaning you are safed