Haskell Servant POST FormUrlEncoded for (Vector String) field

Question

I have a data type

data Reading = Reading
  { title :: String
  , authors :: Vector String
  , startDate :: Day
  , endDate :: Maybe Day
  }
  deriving (Eq, Show, Generic, FromRow, ...)

and I want to be able to create at a POST endpoint for a form

    :<|> "reading" :> ReqBody '[FormUrlEncoded] Reading :> Post '[HTML] Reading

To do this, I need to derive a FromForm instance for Reading. The issue is that there is no instance FromHttpApiData (Vector String). There does seem to be an instance for [String]. I chose Vector String so that I could also derive FromRow, which allows me to naturally pull records using postgresql-simple.

So, is there a type that I can use for the authors field to derive both FromRow and FromForm? I did spend a fair amount of time in the docs and experimenting to no avail. Or maybe I should be looking a defining a custom instance, or parameterizing the type of authors based on the use case?

Answer 1

It does indeed look like there isn't machinery for deriving the FromForm instance for Vector.

I would define the instance by hand:

instance FromForm Reading where
  fromForm f =
    Reading
      <$> parseUnique "title" f
      <*> (fromList <$> parseAll "authors" f)
      <*> parseUnique "startDate" f
      <*> parseMaybe "endDate" f