Perfect Steve, many thanks for the explanation. But just to be sure I
understand,
the multiple eval of input expression, your begin println("hello"); 3 end
would only
occur during macro expansion?
Also, just to beat this poor dead horse into the ground, to get the
behavior I wanted,
get rid of the splice, get rid of the splat and pass a single vector
parameter to the
macro and then eval it there. Now that's the behavior I wanted but
performance is
another issue. How would I reason about the relative performance here?
macro hornervec(x, c)
p = eval(c)
ex = esc(p[end])
for i = length(p)-1:-1:1
ex = :($(esc(p[i])) + t * $ex)
end
Expr(:block, :(t = $(esc(x))), ex)
end
On Saturday, August 30, 2014 12:42:11 AM UTC-7, Steven G. Johnson wrote:
>
> The answer is related to your splicing questions. What gets passed to the
> macro is not the value of the argument, but rather the symbolic expression
> of the argument. If I didn't use a temporary variable, that symbolic
> expression would get inserted multiple times into the polynomial
> evaluation. This is not what you want because it means the expression
> could be evaluated multiple times.
>
>>
>> Try passing an expression with a side effect and you'll see what I mean:
>>
>> @horner(begin
>> printf("hello")
>> 3
>> end, 4,5,6,7)
>>
>
> Whoops, I mean println, not printf. And I mean, try passing it to a
> version of horner that does not use a temporary variable.
>
