Warn against using 'fail' directly in user-code

[Ganesh Sittampalam]

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