st: How to pass scalar argument to a function's options (-prvalue- in this case)

[Richard Herron <richard.c.herron@gmail.com>]

I think this is a gross conceptual error I have in how Stata uses
scalars, but I have an example using Long and Xu's -prvalue-.

I would like to evaluate a -probit- model with a logged right hand
side variable at the "log of the mean" instead of at the "mean of the
log." I find the mean with -summarize- then store the log with
-scalar-, but when I pass -log_mean_weight- as an argument to the
-x()- option in -prvalue- I get the following error

'log_mean_weight' found where number expected

Here is some reproducible code

* begin code
sysuse auto, clear
generate log_weight = log(weight)
generate price_dummy = (price > 10000)
probit price_dummy log_weight mpg length

* I would like to determine probability at the log of mean weight
summarize weight, meanonly
scalar log_mean_weight = log(r(mean))
prvalue, x(log_weight = log_mean_weight)
* end code

I have also unsuccessfully tried to use a -tempname- instead of
-scalar-, but in that case -prvalue- returns

'__000000' found where number expected

Thanks! I think (well, hope) that this scalar problem is my last major
Stata blind spot.
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