> On the other hand if types are annotated with the exceptions they
> throw then they become just another return value and become pure.
> Same input, same exception every time.
>

As far as Yotam Barnoy's goal is concerned, this isn't true. He wants to be
able to use purity annotations for optimisation: a most typical
optimisation which is valid only for pure functions is to rewrite *map f
(map g l)* into *map (f∘g) l*. This optimisation, as it happens, is not
valid with exception throwing functions as it may very well throw a
different exception.