Re: [Haskell-cafe] Sparse records/ADTs

* Roman Cheplyaka [2012-10-24 13:55:17+0300]

* Jon Fairbairn [2012-10-24 11:08:29+0100]

...

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?

For runtime efficiency it's better to use Data.Map.

Actually, you can use Data.IntMap for even better performance, if you define an Enum instance for your keys. Roman