You've got a bunch of great answers, if there's no rhyme or reason to
which fields are missing.
If, on the other hand, they will tend to be present or absent in
groups, you could decompose your data-structure a bit, for fast
lookups, good space efficiency, and maybe even slightly more
interesting checks from the type system.
On Wed, Oct 24, 2012 at 3:08 AM, 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@cl.cam.ac.uk
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