[Haskell-cafe] Re: Are there standard idioms for lazy, pure error handling?

[Henning Thielemann]

Somehow I missed this thread. I want to say that I have implemented a general 
way to add the information of an exception to the "end" of an arbitrary data 
structure.

http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/html/Control-Monad-Exception-Asynchronous.html


Duncan already mentioned it:

> data Exceptional e a = Exceptional {
>   exception :: Maybe e
>   result    :: a
> }

> However it's pretty clear from the structure of this type that it cannot
> cope with lazy error handling in sequences. If you try it you'll find
> you cannot do it without space leaks.

Actually you spotted the space leak - but how is it pretty clear, that it 
has the space leak? I wondered why 'unzip' does not have the space leak, 
although when I sequence a lot of Exceptional values using mconcat, I'm 
doing something pretty similar.

I also agree with you, that the possibility to access the result directly, 
and thus easily ignore the exception is bad. I thought it could be 
improved by removing the field accessors and instead provide the function:

switch :: (Maybe e -> c) -> (a -> c) -> Exceptional e a -> c