Extracting database field values inside a Handler

问题

I would like to extract a database field (Text) and pass it as an argument to another function from a Handler. However, I run into Type errors. Completely made up example so may feel a bit contrived but should illustrate the issue I am having.

Person
  name Text

Car
  personId PersonId 
  name Text
  type Text

I would like to get a Car entity and then find the corresponding Person. Get his name and then pass it as an argument. Something like:

data MyD = MyD { input1 :: Int} 

entryForm :: Text -> Form MyD  -- Update: Removed the incorrect extra parameter
entryForm n1 = renderDivs $ MyD
  <$> areq intField n1 Nothing

My get handler looks like:

getInputR :: CarId -> Handler Html
getInputR carId = do  
  car <- runDB $ get404 carId
  pid <- carPersonId car
  name <- getPName pid
  (widget, enctype) <- generateFormPost $ entryForm name
  defaultLayout $ do
     $(widgetFile "my_template")
  where
     getPName pid = do
         person <- runDB $ get404 pid
         personName person

I get an error saying:

Couldn't match expected type `HandlerT App IO t0'
       with actual type `KeyBackend
                      persistent-1.2.1:Database.Persist.Sql.Types.SqlBackend Person'
In the return type of a call of `carPersonId'
In a stmt of a 'do' block: pid <- carPersonId car

What am I doing wrong?

Thanks!

回答1:

Try changing

pid <- carPersonId car
name <- getPName pid

name <- getPName $ carPersonId car

The value returned from your runDB call is not inside the handler monad so you don't need to use the arrow syntax to access it.

For the second error, the issue is similar: The getPName function's return type is in the Handler monad since it uses runDB, so you need to use return to put the value into the monad:

getPName pid = do
     person <- runDB $ get404 pid
     return $ personName person

来源：https://stackoverflow.com/questions/17891488/extracting-database-field-values-inside-a-handler

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haskell

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