MaybeT.hs

-- original
getBody :: ServerPart L.ByteString

parseBody :: FromJSON a => ServerPart (Maybe a)
parseBody = getBody >>= (return . A.decode)

-- attempt
parseBody :: FromJSON a => MaybeT (ServerPartT IO) a
parseBody = lift $ getBody >>= (return . A.decode)
-- Error
-- Couldn't match type ‘a’ with ‘Maybe a0’
--     ‘a’ is a rigid type variable bound by
--         the type signature for
--           parseBody :: FromJSON a => MaybeT (ServerPartT IO) a
--         at src/Main.hs:65:14
--   Expected type: L.ByteString -> a
--     Actual type: L.ByteString -> Maybe a0
--   Relevant bindings include
--     parseBody :: MaybeT (ServerPartT IO) a (bound at src/Main.hs:66:1)
--   In the second argument of ‘(.)’, namely ‘decode’
--   In the second argument of ‘(>>=)’, namely ‘(return . decode)’

-- So Maybe a0 doesn't match FromJSON a, that I get, but I was trying to use MaybeT (ServerPartT IO) a 
-- as a replacement for ServerPartT IO (Maybe a), so there's something I'm not quite understanding