Henning,
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")
If this is an actual use case, then there is something very wrong. Handling non-empty lists can be done cleanly using various non-empty list types. Or you avoid 'head' by using 'case' or 'viewL'. If instead you plan to catch something then you should use Either, ExceptionalT, ErrorT or IO exceptions. I am afraid that an Eq instance for ErrorCall promotes the abuse of 'error'.
Please don't get me wrong. Personally I always prefer pattern matching over /exception throwing functions/. However, I still think making it hard to "deal" with exception throw function is the wrong approach. Regarding testing: Failure behavior is part of the public interface of a function. If a function throws an exception (whether I like it or not), I'd like to have it documented. The best way to document something is a test case (IMHO). Cheers, Simon