[REBOL] Re: The evaluation semantics
From: lmecir::mbox::vol::cz at: 3-Sep-2005 23:10
Volker Nitsch napsal(a):
>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
>
As I said, there *are* exceptions:
abc: func [:x] [abs x]
abc -4 + -5 ; == -1
-L
