'any' and 'all' compared with the rest of the Report

Jan-Willem Maessen jmaessen@mit.edu
Thu, 25 Jan 2001 11:09:07 -0500

Bjorn Lisper <lisper@it.kth.se> replies to my reply:
> >My current work includes [among other things] ways to eliminate this
> >problem---that is, we may do a computation eagerly and defer or
> >discard any errors.
> What you basically have to do is to treat purely data-dependent errors (like
> division by zero, or indexing an array out of bounds) as values rather than
> events. 

Indeed.  We can have a class of deferred exception values similar to

> Beware that some decisions have to be taken regarding how error
> values should interact with bottom. (For instance, should we have
> error + bottom = error or error + bottom = bottom?) The choice affects which
> evaluation strategies will be possible.

Actually, as far as I can tell we have absolute freedom in this
respect.  What happens when you run the following little program?

forever x = forever x

bottomInt :: Int
bottomInt = error "Evaluating bottom is naughty" + forever ()

main = print bottomInt

I don't know of anything in the Haskell language spec that forces us
to choose whether to signal the error or diverge in this case (though
it's clear we must do one or the other).  Putting strong constraints
on evaluation order would cripple a lot of the worker/wrapper-style
optimizations that (eg) GHC users depend on for fast code.  We want
the freedom to demand strict arguments as early as possible; the
consequence is we treat all bottoms equally, even if they exhibit
different behavior in practice.  This simplification is a price of
"clean equational semantics", and one I'm more than willing to pay.

-Jan-Willem Maessen

[PS - I've deliberately dodged the issue of program-level exception
handling mechanisms and the like.]