Since base 4.7, there is a new exception "superclass" SomeAsyncException which all asynchronous exceptions are supposed to extend (via the extensible exceptions mechanism). Adding support for this to syncIO would do two things:

• Add support for user-defined async exceptions
• Make for a more universal definition of async exceptions

Looking at the list of exceptions currently marked as asynchronous, it looks like many of them (like ArithException) are miscategorized, which can lead to incorrect behavior, e.g.:

#!/usr/bin/env stack
-- stack --resolver lts-6.3 runghc --package unexceptionalio

import Control.Exception
import UnexceptionalIO

main :: IO ()
main = syncIO (evaluate (1 `div` 0)) >>= print

Results in a runtime exception:

async.hs: divide by zero

By contrast, with safe-exceptions:

import Control.Exception (evaluate)
import Control.Exception.Safe

main :: IO ()
main = tryAny (evaluate (1 `div` 0)) >>= print

Results in printing a Left value as expected:

Left divide by zero

Pinging @Gabriel439 as well, since this affects errors.