Proposal: Add Eq instance for Control.Exception.ErrorCall

[Simon Hengel]

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/