Comments on: Data Structure Agnostic JSON Serialization

By: lpsmith

lpsmith — Fri, 24 Feb 2012 18:57:33 +0000

foldMap from Data.Foldable I assume? Thanks for pointing that out to me...

By: lpsmith

lpsmith — Fri, 24 Feb 2012 18:51:30 +0000

That thought has crossed my mind, I think, but I haven’t really looked into it.

By: ceii

ceii — Fri, 24 Feb 2012 16:35:43 +0000

I’ve just spent a few hours looking at ways to abstract this mess away, but I seem to inevitably gravitate toward a bastardized version of parsing combinators.

Looking at that, I guess providing specialized parser combinators directly would be smarter. Have you looked into that?

By: Sjoerd Visscher (@sjoerd_visscher)

Sjoerd Visscher (@sjoerd_visscher) — Fri, 24 Feb 2012 12:20:15 +0000

I’m going to promote my favorite Haskell function again: Instead of “foldr (\x xs -> element x `mappend` xs) mempty”, you can just write “foldMap element”.

By: lpsmith

lpsmith — Fri, 24 Feb 2012 12:06:51 +0000

Actually, I’ve played around with a similar idea; it just doesn’t have the interface I want. I wouldn’t consider it a convenient way to express deserializers, at least, not without something sitting on top of it hiding all the arcana. But maybe you know how to do that?

By: ceii

ceii — Fri, 24 Feb 2012 10:51:45 +0000

What you’re doing is basically making aeson’s Value type virtual. Just like you can allow users to “create” values of that virtual type by defining functions analogous to each constructor, you can allow them to “consume” a value that type by offering a fold operator for it:

type FoldJSon r = (HashMap Text r -> r) -> (Vector r -> r) -> (Text -> r) -> (Integer -> r) -> (Double -> r) -> (Bool -> r) -> (() -> r) -> r

(The () parameter to the second-to-last argument is obviously superfluous, I’ve added it to make it clear that this argument is a continuation for the Nil case.)

You could then implement a deserialization API with the following interface:

class FromJSon a where
parseJSon :: FoldJSon (Parser a) -> Parser a

decode :: FromJSon a => ByteString -> Maybe a

in a way exactly analogous to what aeson does, except with some additional inversion of control and no concrete Value type.

I think the name of this game is “refunctionalization”, but I’m not 100% sure.