Aeson Revisited

As many of you know, the documentation situation in Haskell leaves something to be desired. Sure, if you are enlightened, and can read the types, you’re supposedly good. Personally, I prefer a little more documentation than “clearly this type is a monoid in the category of endofunctors”, but them’s the breaks.

Long ago, I wrote about some tricks I found out about using Aeson, and I found myself using Aeson again today, and I’d like to revisit one of my suggestions.

Types With Multiple Constructors

Last time we were here, I wrote about parsing JSON objects into Haskell types with multiple constructors. I proposed a solution that works fine for plain enumerations, but not types with fields.

Today I had parse some JSON into the following type:

data Term b a = App b [Term b a] | Var VarId | UVar a

I thought “I’ve done something like this before!” and pulled up my notes. They weren’t terribly helpful. So I delved into the haddocs for Aeson, and noticed that Aeson's Result type is an instance of MonadPlus. Could I use mplus to try all three different constructors, and take whichever one works?

instance (FromJSON b, FromJSON a) => FromJSON (Term b a) where parseJSON (Object v) = parseVar `mplus` parseUVar `mplus` parseApp where parseApp = do ident <- v .: "id" terms <- v .: "terms" return $ App ident terms parseVar = Var <$> v .: "var" parseUVar = UVar <$> v .: "uvar" parseJSON _ = mzero

It turns out that I can.

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