Question

I’m aware of partial updates for records like :

data A a b = A { a :: a, b :: b }
x = A { a=1,b=2 :: Int }
y = x { b = toRational (a x) + 4.5 }

Are there any tricks for doing only partial initialization, creating a subrecord type, or doing (de)serialization on subrecord?

In particular, I found that the first of these lines works but the second does not :

read "A {a=1,b=()}" :: A Int ()
read "A {a=1}" :: A Int ()

You could always massage such input using a regular expression, but I’m curious what Haskell-like options exist.

Answer 1

Partial initialisation works fine: A {a=1} is a valid expression of type A Int (); the Read instance just doesn’t bother parsing anything the Show instance doesn’t output. The b field is initialised to error "...", where the string contains file/line information to help with debugging.

You generally shouldn’t be using Read for any real-world parsing situations; it’s there for toy programs that have really simple serialisation needs and debugging.

I’m not sure what you mean by “subrecord”, but if you want serialisation/deserialisation that can cope with “upgrades” to the record format to contain more information while still being able to process old (now “partial”) serialisations, then the safecopy library does just that.