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

You want extensible records which, I gather, is one of the most talked about topics in Haskell. It appears that there is not currently much consensus on how to implement it.

In your case it seems like maybe instead of an ordinary record you could use a heterogeneous list like those implemented in HList.

Then again, it seems you only have two levels here: common and program. So maybe you should just define a common record type for the common options and a program-specific record type for each program, and use StateT on a tuple of those types. For the common stuff you can add aliases that compose fst with the common accessors so it's invisible to callers.