On Mon, 2 May 2022, Ivan Krylov wrote:
Hi,
I've been playing around with formals() and stumbled upon some
behaviour which surprised me:
a <- alist(x=)
.Internal(inspect(a$x))
# @55b0ebae67b0 01 SYMSXP g0c0 [MARK,REF(5761)] "" (has value)
x <- a$x
.Internal(inspect(x))
# Error: argument "x" is missing, with no default
missing(x)
[1] TRUE
Apparently, the elements corresponding to formal arguments without a
default value can be passed to functions as arguments (and look like
empty symbols, which can't be created by as.symbol()), but when
assigned to a variable, they turn missing and can't be accessed.
How can I safely detect and work with such values?
You can detect them as you did here using missing(). This is what the
byte code compiler implementation does.
[https://homepage.stat.uiowa.edu/~luke/R/compiler/compiler.pdf]
It's not ideal but it's what we have and not likely to change any time
soon.
Best,
luke
--
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
241 Schaeffer Hall email: luke-tier...@uiowa.edu
Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
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