如何使用scotty从sqlite和响应json获取数据？

Question

I'm trying to build a simple blog using Haskell and the Framework Scotty. Using a Model.hs I have:

data Post = Post
    { id :: Int
    , tipo :: String
    , titulo :: String
    , conteudo :: String
    } deriving (Show, Generic)

I've already create a schema using sqlite and populate with some data, right now I'm trying to get this data using this method in my Storage.hs

selectPosts :: Sql.Connection -> IO [M.Post]
selectPosts conn =
    Sql.query_ conn "select * from post" :: IO [M.Post]

My intent is get the data format as json in my Main.hs:

instance ToJSON M.Post
instance FromJSON M.Post

main :: IO ()
main = do
    putStrLn "Starting Server..."
    scotty 3000 $ do
        get "/" $ file "templates/index.html"
        get "/posts" $ do
            json posts where
            posts = withTestConnection $ \conn -> do
                S.selectPosts conn

But I'm getting an IO [Model Post] and I don't know how to render this as json, so its keeping getting this error:

No instance for (ToJSON (IO [Post])) arising from a use of ‘json’

My project is in github to run just use stack build and after stack ghci. In the building I'm already getting this error.

Answer 1

In Haskell, all functions are pure---so something like selectPosts , which needs to go out and do IO to talk to a database, can't just do that and return the value from the database. Instead, these kinds of functions return something of type IO a , which you can think of as basically a description of how to go out and do IO to get a value of type a . These "IO actions" can be composed together, and one of them can be assigned to main ; at runtime, the RTS will execute these IO actions.

However, you aren't composing the IO a value that you get back from selectPosts to be part of the larger IO value that eventually becomes main ; you are trying to directly use it by feeding it into json . This won't work, because there's no (good/easy) way to convert a description of how to do IO into a JSON string.

The way that Haskell deals with composing these values is through an abstraction known as a "monad", which is useful in a lot of other cases as well. do notation can be used to write this monadic sequencing in a very natural style. You can't just write posts <- withTestConnection S.selectPosts here because Scotty's get function takes a value of the monadic ActionM type, not IO . However, it turns out that ActionM is basically a bunch of other useful stuff layered on top of IO , so it should be possible to "lift" the IO action from selectPosts into Scotty's ActionM monad:

get "/posts" $ do
  posts <- liftIO $ withTestConnection S.selectPosts
  json posts

Side note: You may have noticed that I wrote withTestConnection S.selectPosts instead of withTestConnection $ \\conn -> do S.selectPosts conn . Generally, if you have a do block with only one expression in it (not of the form x <- act ), this is the same as the single expression outside of a do block: \\conn -> S.selectPosts conn . Furthermore, Haskell tends to encourage partial application: you have S.selectPosts , which is a function Sql.Connection -> IO [M.Post] . \\conn -> S.selectPosts conn is another function, of the same type, which passes the connection into selectPosts and then returns the same result as selectPosts ---this function is indistinguishable from selectPosts itself! So if this is all that you need inside your withTestConnection , you should be able to simplify the whole lambda and do block to just S.selectPosts .