calling eval in a macro doesn't do what you think it does, so it doesn't do
what you want
On Sat, Aug 30, 2014 at 5:05 PM, Don MacMillen <don.macmil...@gmail.com>
wrote:
> 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.
>>
>
