Extensible serialization in Haskell -

i'm trying make serialization/deserialization using read , show (which not problem per se), extensible in sense data type can extended (but not shrunk).

suppose have type:

data foo = { bar :: int } deriving (show, read) 

and list:

foos = [foo 1, foo 2] 

i can deserialize file:

hputstrln filehand . ppshow $ foos 

then can serialize back:

!str <- hgetcontents filehand let foosfromfile = frommaybe [] $ (readmaybe :: string -> maybe [foo]) str 

but suppose months later want add 'baz' field foo type. direct serialization old-format file no longer work read, need convert file (which don't want).

so, there elegant (without putting explicit versioning logic in program itself) solution still serialize data file, , filling-in missing fields default values? maybe types tricks?

thanks.

this might not looking since want avoid explicit versioning i'd still point out safecopy go-to solution versioned serialization , @ least makes painless.

i don't think there's way use default show , read instances while supporting adding arbitrary amount of new fields, can of course write own read instance hand handles missing record fields. however, think that's more laborious , error prone using safecopy.