Proposal: Add Eq instance for Control.Exception.ErrorCall
sol at typeful.net
Tue Nov 27 12:37:44 CET 2012
> >I propose to add an Eq instance for ErrorCall. The main motivation is
> >to make it more convenient to construct predicates that select specific
> >My current use case is testing for expected exceptions. In Hspec 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).
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