Parsing a JSON post

问题:

I have the following piece of code:

data Friend = Friend
              { friend_name :: Text
              , friend_inTwitter :: Bool
              , friend_twitterName :: Maybe Text
              }
$(deriveJSON (drop 6) ''Friend)

This piece of JSON is being posted to a handler, and I'm having a difficult time getting it. I've tried different things, but let me just put one of them here to generate suggestions:

postTestR :: Handler RepPlain
postTestR = do
value <- parseJsonBody_
return $ RepPlain $ friend_name value

That doesn't work, and I can see that the types don't match, but I'm not sure what to replace it with. I would also like to see how I could parse a list friends that gets posted as JSON.

Thanks!

回答1:

Well, as it turn out, I needed to add a "toContent" call in there to convert from Text to Content. Here is a piece of code that works:

data Person = Person
              { person_firstName :: Text
              , person_lastName :: Text
              , person_fullName :: Text
              , person_friends :: [Friend]
              }

data Friend = Friend
              { friend_name :: Text
              , friend_inTwitter :: Bool
              , friend_twitterName :: Text
              }


$(deriveJSON (drop 7) ''Person)              
$(deriveJSON (drop 7) ''Friend)   


postKnockoutR :: Handler RepPlain
postKnockoutR = do
  value <- parseJsonBody_
  let (f:fs) = person_friends value
  return $ RepPlain $ toContent $ friend_name f

回答2:

I'm not a Yesod expert, but if you look at the return type of parseJsonBody_, you'll see that it's FromJSON a => GHandler sub master a, not just plain FromJSON a => a. This makes sense: you cannot parse the request body outside a HTTP context in which the request is accessible, and since the HTTP context is implemented as a monad, you'll have to write monadic code to deal with it.

So instead of assignment, you probably want to extract the JSON value from the Handler monad:

value <- parseJsonBody_

In your code, value is of type GHandler sub master Friend, which is not what you want.