On 20/12/2013 23:14, Stijn van Drongelen wrote:
Some monads have no sensible implementation for fail, so therefore, `fail` can't be bound by any laws besides what's in the type. So when you're writing common functions involving error handling, the type of `fail` gives you some requirements on how to use it, but you get no promise that it does something useful (note: `fail = error` is not useful).
However, when you use something like MonadError, you're guaranteed (by the type) that you can use throwError, and promised (by those who create instances for MonadError) that it will have a useful meaning.
(And this is just one of the reasons.)
But as long as it used for pattern match failures, those problems will exist anyway - is using it explicitly any worse than writing incomplete patterns in a do block? Ganesh