[Rd] An argument promise containing bytecode?

Taras Zakharko t@r@@@z@kh@rko @end|ng |rom uzh@ch
Sun Mar 7 11:12:48 CET 2021

Dear all, 

I was playing around with some C code that inspects argument promises and I noticed that forwarded arguments sometimes contain bytecode in the PRCODE component. E.g. if I have a nested call like this:

  f <- function(x) x
  g <- function(x) f(x)


and inspect the value of “x” in g’s frame from within f’s frame (basically calling findVar(parent.frame(), x)), I would see a LANGSXP the first time g()  is invoked, but BCODESXP on every subsequent time.  Using PREXPR gets me the original LANGSXP in any case. 

Now, this is curious behavior and I had a quick glance into R source code related to closure evaluation and argument matching, but I could not see anything that would generate bytecode for promises. Could someone with in-depth knowledge of R’s machinery explain what is going on? 



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