[Haskell] Better Exception Handling

Jules Bean jules at jellybean.co.uk
Wed Nov 24 14:14:28 EST 2004

On 24 Nov 2004, at 18:28, John Goerzen wrote:

> I note, though, that "making an Either into a Monad" doesn't do 
> anything
> to deal with asynchronous exceptions.

We may be talking at cross purposes here. If, by 'asynchronous 
exceptions' you mean that exceptions may lurk arbitrarily deeply within 
user defined data structures then using Either *does* solve this 

Using 'Either' makes it crystal clear exactly where exception values 
can exist. Rather than being arbitrarily deep in the structure, 
exceptions can only exist at the nodes that you choose to decorate with 
Either: which, in the obvious usage pattern, is only the root node.  
This doesn't mean that exceptions can't be *raised* at any depth, it 
just means that deep exceptions are propagated. In other words, the 
'Either' pattern will actually, as a side effect, force complete 
evaluation when you check for exceptions. [To be clear: it will only do 
this if you choose to use the usage pattern where only the root node is 
decorated as 'Either' and your functions propagate the Either-ness]

Writing such code 'by hand' would involve a large amount of wrapping 
and unwrapping (as you observed). That's all "making Either into a 
Monad" will cure: it makes the wrapping and unwrapping of Lefts and 
Rights a product of haskell's clever monadic syntax. It also makes you 
choose an evaluation order, which removes non-determinism (the first 
exception is the propagated one).

If that isn't what you meant by asynchronous exceptions then we are 
indeed talking at cross-purposes!


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