Application letters at the Haskell workshop: suggestion

[Fergus Henderson]

On 15-Sep-2001, Mark Carroll <mark@chaos.x-philes.com> wrote:
> On 14 Sep 2001, Mike Gunter wrote:
> 
> > The problem is not a loss of referential transparency but the
> > requirement that evaluation order must be specified.  E.g.
> > what should
> > 
> >     raise "left" + raise "right"
> > 
> > return? 
> (snip)
> 
> Ah! Yes, I see what you mean - this explains what Andy Moran was thinking,
> probably. (-: Moreover, if we have a pseudo-Haskell code snippet like,
> 
>    try
>      f (a 1) (b 2)
>    catch
>      error-1 -> 1
>      error-2 -> 2
>    where
>      a _ = raise error-1
>      b _ = raise error-2
> 
> ... then is the return value of this 1 or 2? AFAICS a simple way to get
> out of this is to only have one exception type that carries no information
> instead of different ones so we can't distinguish one exception from
> another, but that's obviously not great.

Unfortunately even that doesn't work, because there are still problems with
non-termination.  Consider the following example:

    try
      f (a 1) (b 2)
    catch
      error-1 -> 1
    where
      a _ = raise error-1
      b n = b n

Should this program return 1, or loop?

Giving this program deterministic behaviour requires specifying the order of
evaluation.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.