Question

I am attempting to pattern match on a type in a case statement such as the following:

result <- action
case result of
  Success _ -> do something
  Failure e -> case e of
                 MyException myField -> do take another action
                 _ -> ...

The compiler can’t deduce e ~ MyException which I understand. My question is what other information do I need to supply to the compiler to be able to match my exception type. In this particular case I know that if there is a Failure the returned type will be MyException.

EDIT:

The type of result (From the Aws package) is:
(Transaction r a, ConfigurationFetch (Info r)) =>
Configuration -> r -> IO (Response (ResponseMetadata a) a)

a is from Data.Attempt which is either a Success or Failure.

Answer 1

Assuming you’re using extensible exceptions (which is the default in recent ghc’s), your result is probably something like

data Result = Success MySuccess | Failure SomeException

You need to convert the SomeException to your own exception type. This is done with the function toException :: Exception e => SomeException -> Maybe e. Then you would handle this like:

Failure e -> case toException e of
               Just (MyException myField) -> do take another action
               _ -> ...

Of course this is assuming that I’m right about your Result type.