Haskell record syntax and type classes

Question

Suppose that I have two data types Foo and Bar. Foo has fields x and y. Bar has fields x and z. I want to be able to write a function that takes either a Foo or a Bar as

Answer 1

Seems to me like a job for generics. If you could tag your Int with different newtypes, then you would be able to write (with uniplate, module PlateData):

data Foo = Foo Something Another deriving (Data,Typeable)
data Bar = Bar Another Thing deriving (Data, Typerable)

data Opts = F Foo | B Bar

newtype Something = S Int
newtype Another = A Int
newtype Thing = T Int

getAnothers opts = [ x | A x <- universeBi opts ]

This would extract all Another's from anywhere inside the Opts.

Modification is possible as well.