Proposal: Add Eq instance for Control.Exception.ErrorCall

Hi, I propose to add an Eq instance for ErrorCall. The main motivation is to make it more convenient to construct predicates that select specific exceptions. My current use case is testing for expected exceptions. In Hspec[1] we use predicates for that, e.g.: evaluate (head []) `shouldThrow` (== ErrorCall "Prelude.head: empty list") But I think it can be useful in other situations and it is consistent with the fact that other common exception types have an Eq instance (e.g. ArithException, IOException, ExitCode). Discussion period: 3 Weeks Cheers, Simon [1] http://hspec.github.com/