[REBOL] Re: The evaluation semantics
From: volker::nitsch::gmail::com at: 3-Sep-2005 22:14
On 9/3/05, Ladislav Mecir <[lmecir--mbox--vol--cz]> wrote:
> I was curious what is a more expected precedence for unbiased people,
> and it looks, that the expression:
>
> abs -4 + -5
>
> is rather expected to yield -1 than 9.
>
> Moreover, the evaluation order in Rebol can be called exception-based,
> because in case:
>
> abs -4 + -5
>
> the operator + takes precedence, while in case
>
> -4 + abs -5
>
> the function ABS is evaluated as first. These aren't all the evaluation
> exceptions in Rebol, the quantity of evaluation exceptions is quite high.
>
To me the number of exceptions is quite low:
Rebol looks at the next two values at the same time.
If you have a value followed by an operator,
it is evaluated first. If it is followed by another value+operator,
these are evaluated left to right.
Else it uses the next value, as we are used too.
abs -4 + -5
rebol sees "abs -4". No op, so takes "abs".
Then "-4 +". Operator.
Now it introduces a paren (so to speak).
That gives "abs ( -4 +"
First thing is closing that paren with the -5:
"abs ( -4 + (-5) )"
and that gives 9.
-4 + abs -5
rebol sees "-4 +". introducing paren:
(-4 +
. The right side has no more operators, so rebol sees only "abs".
Normal evaluation. "Abs" needs one arg.
There is "-5 end-of-block". No operator.
So "abs -5". That is "joined":
( -4 + (abs - 5) )
As a result, an operator can "steal" an argument from the left
function. Thus with
abs -4 + -5
the "+" can steal the -4. with
-4 + abs -5
there is nothing which can be stolen from.
That is why rebol-conditions sometimes looks "reversed",
if "volker" = ask "your name?" [..]
instead of
if ask "your name?" = "volker" [..]
The "=" cant steal in the second case.
Thus we need to write
if (ask "your name?") = "volker" [..]
and since we are lazy we save that paren. :)
> -L
>
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--
-Volker
Any problem in computer science can be solved with another layer of
indirection. But that usually will create another problem.
David
Wheeler
