Summary

The typeclass method displayException is not used. This should be remedied by either:

1. Documenting that it does not get used anywhere (implies that there is no point in overriding the default)
2. Using it (in base) when displaying exceptions.

Steps to reproduce

Here is an example that demonstrates that displayException is not used:

import Control.Exception

main :: IO ()
main = throwIO MyException

data MyException = MyException
  deriving (Show,Eq)

instance Exception MyException where
  displayException _ = "custom-my-exception-display"

Running this results in:

$ ./example-exception
example-exception: MyException

Expected behavior

I would expect to see:

amartin@thadtop:~/Development/test$ ./example-exception
example-exception: custom-my-exception-display