oh, that’s reasonably easy then:
macro testfn(expr)
children = esc(:children)
syms = [:($(symbol(string("_", i))) = $children[$i]) for i = 1:10]
return Expr(:let, Expr(:block, esc(expr)), :(children=children), syms..)
end
which will expand to:
let children=children, _1 = children[1], _2 = children[2], ...
$expr
end
(I added the first children=children line, so that the user could
theoretically assign to a variable named children, without leaking it to
the enclosing environment. it's not strictly necessary).
-----
alternatively, you could recurse through their expr code and replace all
variables of the form `_(\d+)` with :(children[\1])
-----
On Fri, Mar 6, 2015 at 9:30 PM Abe Schneider [email protected]
<http://mailto:[email protected]> wrote:
Hmmm, good to know. Thank you.
>
> The rationale for doing so is to provide a shortcut for the elements of a
> variable `children`. Specifically, for a grammar, I might have a rule like:
>
> ```
> @grammar foo begin
> number = r"[0-9]+" { parseint(children[1]) }
> end
> ```
>
> What I would like to have instead, to make it more succint is:
>
> ```
> @grammar foo begin
> number = r"[0-9]+" { parseint(_1) }
> end
> ```
>
> Originally, just to get things working, I used an `eval`, which while
> worked, also made the assignment global, which was less than ideal.
>
>
> I'd be curious if anyone has a suggestion on other methods to accomplish
> this, or if this is outside the scope of what's possible to do with Julia.
>
>
>
> On Friday, March 6, 2015 at 5:10:07 PM UTC-5, Jameson wrote:
>
>> you can't do what you are proposing, by design. a macro cannot do
>> anything that you cannot express directly, it simply allows you to express
>> it more succinctly by templating the redundant parts.
>>
>> if you want a "set" or "numbered list", use a set or number list.
>> variables are bad at that sort of task. whereas an Array is very good at it.
>>
>> On Fri, Mar 6, 2015 at 8:05 AM Abe Schneider <[email protected]> wrote:
>>
>>> I'm trying to create a set of variables (_1, _2, ...) from items within
>>> a list in a macro. I have a (much) condensed version of the code:
>>>
>>> macro testfn()
>>> quote
>>> i = 1
>>> value = [1, 2, 3]
>>> $(Expr(:(=), Expr(:symbol, Expr(:string, "_", :i)), :value))
>>> println(_1)
>>> end
>>> end
>>>
>>>
>>> which gives me:
>>>
>>> ERROR: syntax: invalid assignment location
>>>
>>>
>>> Any ideas on what might be wrong or the proper way to do this? I would
>>> like to keep this in the quotes, as in the actual version there's a lot
>>> more code surrounding the assignment.
>>>
>>> I can get things to work with an eval, but I rather avoid the eval, and
>>> it appears to create a non-local variable.
>>>
>>>
>>> Thanks!
>>>
>>
