[Haskell-cafe] Use unsafePerformIO to catch Exception?

[Donn Cave]

Quoth Duncan Coutts <duncan.coutts at worc.ox.ac.uk>:

> You must not do this. It breaks the semantics of the language.
>
> Other people have given practical reasons why you should not but a
> theoretical reason is that you've defined a non-continuous function.
> That is impossible in the normal semantics of pure functional languages.
> So you're breaking a promise which we rely on.

Could you elaborate a little, in what sense are we (?) relying on it?

I actually can't find any responses that make a case against it on a
really practical level - I mean, it seems to be taken for granted that
it will work as intended, and we're down to whether we ought to have
such intentions, as a matter of principle.  If you've identified a
problem here with semantics that would break normal evaluation, from
the perspective of the programmer's intention, then this would be
the first practical reason?

	Donn

> It is not "safe". It's almost as bad as a function isBottom, which is
> the canonical non-continuous function. It's defined by:
>
> isBottom _|_ = True
> isBottom _   = False
>
> Of course your tryArith only tests for certain kinds of _|_ value, but
> in principle the problem is the same.
>
> It is not safe because it distinguishes values that are not supposed to
> be distinguishable. This invalidates many properties and
> transformations.
>
> Duncan