How to throw exceptions from Handler - haskell

Question

I'm building a RESTful endpoint that should throw an error if there is invalid data. Here's a simplified (and slightly silly) version:
getEventsR :: Handler Value
getEventsR = do
mpage <- lookupGetParam "page"
let filters = case hasPage mpage of
Right val -> val
Left val -> error $ T.unpack val -- Throw an error
-- The rest...
hasPage :: Maybe Text -> Either Text Text
hasPage =
Left "This should error out"
Is this the correct way - a function should return an Either that will later be expanded to an error?
I probably shouldn't use error, right? What instead?
As a side note, I'd be happy if my thrown error would be sent also in JSON format.
Update:
The reason I've used the term "throw exception", is because my code looks something like this (simplified):
let selectOpt = case addPager [] of
Right val -> val
Left val -> error $ T.unpack val
let filters = case addFilter [] of
Right val -> val
Left val -> error $ T.unpack val
events <- runDB $ selectList filters selectOpt :: Handler [Entity Event]
So if it errors out on selectOpt I'd like it to short-circuit, and not continue with the rest of the handler.

Answer 1

You can use the sendResponseStatus to directly specify the response:
getEventsR = do
mpage <- lookupGetParam "page"
case hasPage mpage of
Left message -> sendResponseStatus status400 message
Right val -> ...continue processing with val...
The second argument to sendResponseStatus can be any type which has a ToTypedContent instance. This includes Text, Value and other common types. See this page for types which have this instance defined by default.
Update
In response to your comments...
get404 is defined as:
get404 key = do
mres <- get key
case mres of
Nothing -> notFound'
Just res -> return res
notFound' :: MonadIO m => m a
notFound' = liftIO $ throwIO $ HCError NotFound
HCError is a constructor of the HandlerContents type:
http://hackage.haskell.org/package/yesod-core-1.4.0/docs/Yesod-Core-Types.html#t:HandlerContents
data HandlerContents:
HCContent Status !TypedContent
HCError ErrorResponse
HCSendFile ContentType FilePath (Maybe FilePart)
HCRedirect Status Text
HCCreated Text
HCWai Response
HCWaiApp Application
An ErrorResponse has the following constructors:
data ErrorResponse:
NotFound
InternalError Text
InvalidArgs [Text]
NotAuthenticated
PermissionDenied Text
BadMethod Method
So you can throwIO a limited number of errors, and the error messages are limited to Text.