Why aren't Persistent types instances of ToJSON/FromJSON in Yesod?

Question

It's not that hard to write ToJSON/FromJSON instances for the generated types but still, while you're generating code could you throw that in? Or is there an easy way to make this happen as a Yesod user? (I haven't dug too deep into how TH works...)

Update: OK, I like this suggestion but, say my persistent type is User. If I use

$(deriveJSON id ''User)

it yields

  Exception when trying to run compile-time code:
  Data.Aeson.TH.withType: Unsupported type: TySynD Model.User [] (AppT (ConT Model.UserGeneric) (ConT Database.Persist.GenericSql.Raw.SqlPersist))
  Code: deriveJSON (id) 'User

apparently because it's an alias. But

$(deriveJSON id ''UserGeneric)

yields

Kind mis-match
The first argument of `UserGeneric' should have kind `(* -> *)
                                                      -> *
                                                      -> *',
but `backend[i5XB]' has kind `*'

I've probably still got the wrong type but I can't find enough about what Persistent generates to get the right incantation.

Answer 1

I actually think we'll be adding this feature to Persistent 0.8 (to be released with Yesod 0.10 in a week or two). It's true what dflemstr said about dependency bloat, which is why we haven't done this in the past, but we already depend on aeson now for our configuration types (based on Yaml config files, which uses aeson's data types).

Answer 2

For anyone who doesn't notice the sub comment on Michael Snowman's post, in resent versions of persistent you can do:

Person json
    name Text
    age Int

and get ToJSON and FromJSON instances of Person.

Answer 3

You can just use the automatic deriving mechanism in Data.Aeson.TH.

{-# LANGUAGE TemplateHaskell #-}
$(deriveJSON id ''Foo)

This should work fine on both Yesod-generated data types as well as your own types.

It takes a function to customize the record field names. Here, I've just passed id to get them unchanged. See the documentation for details.

Answer 4

The default Yesod type generators shouldn't generate ToJSON/FromJSON instances, because that would add a dependency on aeson even if you didn't want to use that package, which could lead to dependency bloat.

You can import Data.Aeson.TH and use this code to automatically create JSON instances:

data MyDataType = ...

deriveJSON id ''MyDataType

Replace id with a function that renames fields for you, if you don't want the same field names in Haskell as in the JSON file.