[Haskell-cafe] testing for exceptions

[joachifm at fastmail.fm]

The ability to express non-termination is a feature, not a bug. If the
program truly cannot produce a useful result for some input, it should
crash, the earlier the better. Wrapping the return value ONLY to make a
non-total program *appear* total is kind of ugly (and commits you to a
potentially inappropriate/nonsensical model for the problem at hand). It
is cleaner to either constrain the input to values you're prepared to
deal with or crash when the implicit invariant is violated.