[Haskell-cafe] Sparse records/ADTs

[Jon Fairbairn]

Is there a convenient way of handling a data structure with lots
of fields of different types that may or may not be filled in?

Something equivalent to

data D = D {a::Maybe A, b::Maybe B, c::Maybe C, …}

but with better space efficiency and a more convenient empty
object.

An easy alternative is

data E = Ea A | Eb B | Ec C | …
type R = [E]

which has a straightforward empty object, but one then must
define

   getA e = listToMaybe [a | Ea a <- e]

for each field, which is tedious (and O(n)). Obviously Templates
would help, but is there an alternative I’ve missed?

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk