There are a number of other data types in that module that have a similar construction IIRC, such as AssertionFailed, NoMethodError, PatternMatchFail, RecConError, RelSelError, etc. which should probably pick up Eq, Ord for consistency (and become newtypes).
and a few others that can have the obvious trivial, empty Eq, Ord like NonTermination, NestedAtomically, BlockedIndefinitelyOnMVar.
The module is currently a bit ad hoc about which types support Eq and Ord. For instance, ArrayException already supported this kind of Eq/Ord, and all the multiple constructor versions did, but none of the single-constructor trivial versions and only some of the reasonable-to-newtype ones.
-Edward
On Tue, Nov 27, 2012 at 8:57 AM, Simon Marlow
<marlowsd@gmail.com> wrote:
On 27/11/12 10:59, Simon Hengel wrote:
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
http://www.haskell.org/pipermail/cvs-libraries/2012-October/016043.html
Cheers,
Simon