Aeson's deriveJSON doesn't work as expected for enums

Question

I usually write my own ToJSON and FromJSON instances, but I decided to use deriveJSON for a type because it was so simple:

data Priority = HIGH | MEDIUM | LOW deriving Show
$(deriveToJSON id ''Priority)

main = BS.putStrLn . encode $ HIGH

I would've expected the JSON derivation to just write out the enum as a string. Instead, it's the key to a hash! {"HIGH":[]}

Why is this the default behavior?

Answer 1

It's because aeson doesn't distinguish between sum types like your Priority and more sophisticated types like data PriorityAndDetails = HIGH { highReason :: Text, alertType :: Alert } | MEDIUM { personResponsible :: Person } | LOW. Fundamentally each of these types are "just a data constructor with n arguments".

In Priority HIGH, MED, and LOW are each just data constructors with zero arguments. In PriorityAndDetails HIGH, MED, and LOW are each just data constructors with some number of named arguments, 2, 1, and 0 respectively.

Generally I've found that you're likely to need to create your own ToJSON and FromJSON instances for anything besides early prototyping.