Yesod Forms or JSON/Ajax or Other

Question

I'm writing a CRUD interface for my Yesod-based site, but I'm having a problem with some values. In particular, I am trying to CRUD on values like:

data Value = Value { valueId :: UUID
                   , otherStuff :: Stuff
                   }

I am having trouble figuring out a good way to create one of these using the tools Yesod and Aeson provide. When I used formlets, in the distant past, I would use the monad transformer version of the form to do something along the lines of:

Value <$> (liftIO getUUID)
      <*> textField Nothing

(I'm probably not getting the formlets syntax exactly right, but hopefully the Haskellers out there get the gist). This had the effect of making a form that did not include a UUID field and a form parser that ran the IO action to make the data at post-time.

I'm not sure how to proceed. I considered the same kind of idea with Aeson parsing, but attoparsec (the underlying parser) is not a monad transformer either.

Can anybody think of any more options other than having an intermediate data representation?

Answer 1

If what you're trying to do is call an IO action in an Applicative form, there is a trick you can use:

lift (liftIO action)

The issue is that an Applicative can't be an instance of MonadIO, but it can be an instance of MonadTrans.