[Haskell-cafe] Sparse records/ADTs

Jon Fairbairn jon.fairbairn at cl.cam.ac.uk
Wed Oct 24 12:08:29 CEST 2012

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

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

   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

More information about the Haskell-Cafe mailing list