`wreq` Get / Post with exception handling - haskell

Question

I am making some http calls using wreq and would like to catch any exception and return an Either type. I tried something like this but could not figure out how to manipulate the calls so it will type check.
-- exhaustive pattern match omitted here
safeGetUrl :: URL -> Maybe Login -> Maybe Password -> IO (Either String (Response LBS.ByteString))
safeGetUrl url (Just login) (Just pass) = do
let def = defaults
opts = def & auth ?~ basicAuth (BS.pack login) (BS.pack pass)
r <- getWith opts url `E.catch` handler
return $ Right r
where
handler :: HttpException -> Either String (Response LBS.ByteString)
handler (StatusCodeException s _ _) = do
return $ Left $ LBS.unpack (s ^. statusMessage)
I am pasting the type error below but I know the above code will not compile. The issue is r <- getWith opts url E.catch handler. The first part returns IO (Res... but the exception handler returns Either... I tried adding lifting the getWith.. into Either but that did not type check either.
Couldn't match type ‘Either String (Response LBS.ByteString)’
with ‘IO (Response LBS.ByteString)’
Expected type: HttpException -> IO (Response LBS.ByteString)
Actual type: HttpException
-> Either String (Response LBS.ByteString)
In the second argument of ‘catch’, namely ‘handler’
In a stmt of a 'do' block: r <- getWith opts url `catch` handler
Is there a way to catch this exception and return an IO Either type?

Answer 1

Since #jozefg answer, the API has changed a little bit and the answer doesn't compile anymore.
Here is an updated version that compiles:
import qualified Control.Exception as E
import Control.Lens
import qualified Data.ByteString.Char8 as BSC
import qualified Data.ByteString.Lazy as LBS
import Network.HTTP.Client
import Network.Wreq as NW
type URL = String
type Login = String
type Password = String
safeGetUrl ::
URL
-> Maybe Login
-> Maybe Password
-> IO (Either String (Response LBS.ByteString))
safeGetUrl url (Just login) (Just pass) = do
let def = defaults
opts = def & auth ?~ basicAuth (BSC.pack login) (BSC.pack pass)
(Right <$> getWith opts url) `E.catch` handler
where
handler :: HttpException -> IO (Either String (Response LBS.ByteString))
handler (HttpExceptionRequest _ (StatusCodeException r _)) =
return $ Left $ BSC.unpack (r ^. NW.responseStatus . statusMessage)

Answer 2

Your issue is that one side of the handle returns an unwrapped response (no Either) and the other side returns an Either-wrapped exception. You then attempt to wrap the response in an Either, which you do need to do, but it's just at the wrong place. You can fix this merely by switching where you do the wrapping
safeGetUrl :: URL -> Maybe Login -> Maybe Password -> IO (Either String (Response LBS.ByteString))
safeGetUrl url (Just login) (Just pass) = do
let def = defaults
opts = def & auth ?~ basicAuth (BS.pack login) (BS.pack pass)
(Right <$> getWith opts url) `E.catch` handler
where
handler :: HttpException -> IO (Either String (Response LBS.ByteString))
handler (StatusCodeException s _ _) = do
return $ Left $ LBS.unpack (s ^. statusMessage)
However there are some other problems with your functions, remember that unpack gives back Word8s not Char. You may want to import Data.ByteString.Char as the version of unpack defined in there should work better than LBS.unpack. Without your imports though I cannot confirm this definitively. The final (working) code for me is
import Control.Lens
import Network.Wreq
import Network.HTTP.Client
import qualified Control.Exception as E
import qualified Data.ByteString.Char8 as BSC
import qualified Data.ByteString.Lazy as LBS
type URL = String
type Login = String
type Password = String
safeGetUrl :: URL
-> Maybe Login
-> Maybe Password
-> IO (Either String (Response LBS.ByteString))
safeGetUrl url (Just login) (Just pass) = do
let def = defaults
opts = def & auth ?~ basicAuth (BSC.pack login) (BSC.pack pass)
(Right <$> getWith opts url) `E.catch` handler
where
handler :: HttpException -> IO (Either String (Response LBS.ByteString))
handler (StatusCodeException s _ _) = do
return $ Left $ BSC.unpack (s ^. statusMessage)