On Tue, Aug 24, 2010 at 05:12, John Lato
Oleg included the error state to enable short-circuiting of computation, and I guess everyone just left it in. Recently I've been wondering if it should be removed, though, in favor requiring explicit (i.e. explicit in the type sig) exceptions for everything. I'm not sure if that would be more or less complicated.
The first few iterations of the "enumerator" package use explicit error types, but this makes it very difficult to combine enumerator-based libraries. The only way I could get everything to work correctly was to always set the error type to SomeException, which is equivalent to and more verbose than making SomeException the only supported type. I believe relying on the MonadError class would have the same drawback, since (as far as I know) it's not possible to create MonadError instances which can accept multiple error types.