[R] expression evaluation during recursion

[Duncan Murdoch]

On 22/10/2015 10:20 AM, david.kaethner at gmail.com wrote:
> Hello,
> 
> I’m trying to solve an exercise, where I want to walk through the search path recursively (http://adv-r.had.co.nz/Environments.html <http://adv-r.had.co.nz/Environments.html>). 
> 
> I’m puzzled by a certain behavior and hope somebody can give me an explanation.
> 
> This code works:
> 
> listenv <- function(env = parent.frame()) {
>   if (identical(env, emptyenv())) {
>     #stop("reached emptyenv", call. = FALSE)
>     return(env)
>   } else {
>     print(env)
>     listenv(parent.env(env))
>   }
> }
> 
> Here, the calling environment is determined with a default parameter in the function’s formals. 
> 
> However, if I want to assign the calling environment within the function’s body, I get the error message „infinite recursion“. Also, I never get actual environments (with attributes, that is), only memory addresses like this: <environment: 0x10da46630>.

I'm not sure what you were looking for, but "<environment: 0x10da46630>"
is the normal way to print an environment, unless it happens to be one
of the special named ones (like .GlobalEnv).

> 
> listenv <- function(env) {
>   env <- parent.frame()
>   if (identical(env, emptyenv())) {
>     #stop("reached emptyenv", call. = FALSE)
>     return(env)
>   } else {
>     print(env)
>     listenv(parent.env(env))
>   }
> }
> 
> Any explanation of what’s going on here would be greatly appreciated. I suspect it has to do with when exactly the parent.frame()-expression is evaluated, but that’s not an actual explanation.


Your function completely ignores the "env" argument.  It never recurses.
 In the first case, "parent.frame()" is only a default value, so
recursion happens properly.  If you change the first line in the body to
these two lines

  if (missing(env))
    env <- parent.frame()

it would be equivalent.

Duncan Murdoch