On Mon, 8 Jun 1998, S. Alexander Jacobson wrote:
> 1. it is not logically consistent to treat exceptions as return values
A function cannot do anything but return a value, can it?
> For example, suppose that we define a new function:
>
> > foo' a b = a + b -- foo' is strict in its arguments
>
> Our intuition is that foo' is commutative. foo' a b = foo' b a.
> But that turns out not to be true when you have exceptions.
That's the problem with intuitions: they can be wrong...
Anyhow, if one is to have exceptions procteting +, I don't think that
commutativity of foo' is reasonable: to handle exceptions, you have to do
checks, and that you can only do in one order or another.
> Take x and y from before,
>
> > z = foo' x' y'
>
> What is the value of z? Haskell does not promise to evaluate arguments in
> any particular order so, depending on implementation, z may be either
> Exception DivideByZero or Exception NotFactorialDomain -1.
Actually, using a monad to manage exceptions you can (maybe, have to)
choose a definite order of evaluation of non-exceptionality-conditions.
> Truly exceptional conditions are those that truly are outside of the
> domain of the function being evaluated. e.g. factorial -1
> The VALUE of (factorial -1) is not an exception. Neither is the value of
> (factorial (1 `div` 0)).
> When a function is passed bad arguments, it is not meaningful (from a
> functional perspective) to have it return a value.
In a typed language, a function *cannot* be applied to something outside
its domain. That's the whole point!
> The value of a function over arguments outside its domain is undefined.
> When such an event occurs, the logically consistent behavior is to exit
> function evaluation and tell the caller what was wrong with the
> arguments passed (to the extent it is possible to do).
One can rightfully argue that, if one is willing to consider bottom (which
is a value we cannot test for!) a return value, which we are, considering
an exception a return value is *very* consistent.
-- m
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Mariano Suarez Alvarez The introduction of
Departamento de Matematica numbers as coordinates
Universidad Nacional de Rosario [...] is an act of violence
Pellegrini 250 A. Weyl
2000 Rosario - Argentina
e-mail: [EMAIL PROTECTED]
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