[Haskell-cafe] Re: Exception handling when using STUArray

[Ben Franksen]

Luke Palmer wrote:
> On Wed, Mar 12, 2008 at 4:45 PM, Donn Cave <donn at avvanta.com> wrote:
>>  Well, the problem inherently requires a certain order of
>>  evaluation.  But if you will just handle pattern match failure
>>  in the IO monad, then you can write a simple functional
>>  expression of the problem instead,
>>
>>      let (i, s') = decodeInt s in
>>           let (v, _) = decodeInt s' in
>>                (i, v)
>>
>>  ... where I think `i' can be evaluated without forcing unnecessary
>>  evaluation of v.  It's clearer, and avoids unnecessary strictness!
> 
> Unless of course you don't have an IO monad handy.
> 
> The issue is that exception handling semantics do induce an order of
> evaluation for determinacy: if both functions in a composition throw
> an exception at some point (say in the 3rd element of a list they're
> generating), you need to decide which exception ends up being thrown,
> and to do that based on the lazy evaluation order would break
> referential transparency.
> 
> It bugs me a little how good Haskell is with laziness, but how bad it
> gets when you need laziness with a slightly different structure.  I
> don't see any way it could reasonably be "fixed" if I were designing
> my own language, it's just unsettling.

Isn't this what "imprecise exeptions" were invented (and implemented in GHC)
for? See
http://research.microsoft.com/Users/simonpj/Papers/imprecise-exn.htm

Cheers
Ben