[Haskell-cafe] Sparse records/ADTs

Roman Cheplyaka roma at ro-che.info
Wed Oct 24 12:55:17 CEST 2012

* Jon Fairbairn <jon.fairbairn at cl.cam.ac.uk> [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.

For keys of this map you have two alternatives: either define

    data Key = Ka | Kb | Kc | ...

or, to prevent this duplication at the cost of less convenient notation,
you can do something like

    -- Identity combinator; or use one from Control.Monad.Identity
    newtype I a = I a

    -- Constant combinator
    newtype K a b = K a
      deriving (Eq, Ord)

    data E c = Ea (c A) | Eb (c B)

    deriving instance Eq (E (K ()))
    deriving instance Ord (E (K ()))

    type R = Map.Map (E (K ())) (E I)

When you set a value, you infer the key from the value. (This inference
needs to be written manually or generated.)

When you get a value by the key, you check that the returned constructor
is what you expect and throw an error otherwise (but the latter should
never happen if you maintain the invariant).


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