[Haskell-cafe] Use unsafePerformIO to catch Exception?

[Duncan Coutts]

On Mon, 2009-03-23 at 08:11 -0400, Xiao-Yong Jin wrote:
> Hi,
> 
> I just feel it is not comfortable to deal with exceptions
> only within IO monad, so I defined
> 
> > tryArith :: a -> Either ArithException a
> > tryArith = unsafePerformIO . try . evaluate

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.

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