Editing Handling errors in Haskell (section)

== Exception ==

An unexpected code path, one that rarely but can happen and can be handled if needs be. Typically caused by IO going wrong in some way, like the machine running out of swap and your program terminating, a file not existing, etc. The most basic functions are: 
* <hask>throw :: Exception e => e -> a </hask>
* <hask>try :: Exception e => IO a -> IO (Either e a) </hask>
from [http://hackage.haskell.org/package/base/docs/Control-Exception.html Control.Exception].

Say you were writing a library to do things on reddit, you would define an exception type in your API:

<haskell>
data RedditException
  = Couldn'tUpvote
  | CommentFailed
  | LoginFailed !Text
  | ConnectFailure !HttpError
  deriving (Show,Typeable)

instance Exception RedditException

login :: Details -> IO ()
login details = do
  code <- tryLogin details
  case code of
    (200,val) -> setLoginContext val
    (_,err)   -> throw (LoginFailed err)
</haskell>

Then later you might write <hask>try (login …)</hask> or <hask>catch (login …) (\(e :: RedditException) -> …)</hask> to handle the exception, if needed. Another exception might be a connection failure.

See [http://hackage.haskell.org/package/base/docs/Control-Exception.html Control.Exception] for more detail and related functions.