R eval has misleading documentation for the case that the envir argument is list or pairlist

eval, metaprogramming, r

Solution

For this specific example, in the first case, the definition of `f3` is:

f3
#function(){
#     eval(quote(y), envir = list())
#   }
#<environment: 0x02a41584>

The default `enclos = parent.frame()` argument will be evaluated in the evaluation frame (i.e. "inside") of `eval` -- the parent of `eval`s current environment is the current `environment()` of its wrapper function (`f3`). And its wrapper function "remembers" where it was created and searches for the `y` correctly to find it.

In the second case, `f3` is defined as:

f3
#function(){
#     eval(quote(y), envir = list(),
#          enclos = parent.frame())
#   }
#<environment: 0x02a4e5e4>

Here, `enclos = parent.frame()` is evaluated in the evaluation `environment()` of `f3` and, upon calling `f3()`, its parent is the `.GlobalEnv` where there is no `y`.

As a clearer example (yet equally nested as the example) we could consider:

f0 = function(e) print(e)
f1 = function(e = parent.frame()) f0(e)
fA = function() #returns a function
{
    function() 
    { 
        print(parent.frame())
        print(environment())
        f1() 
    }
}   
fB = function() # returns a function
{
    function() 
    { 
        print(parent.frame())
        print(environment())
        f1(parent.frame()) 
    }
}   

And call:

fA()()
#<environment: R_GlobalEnv>  #<- parent of `fA()`'s `environment()`
#<environment: 0x06ff25bc>   #<- current env of `fA()`
#<environment: 0x06ff25bc>   #<- parent of `f1` == current of `fA()`

fB()()
#<environment: R_GlobalEnv>  #<- parent of `fA()`'s `environment()`
#<environment: 0x06fec304>   #<- current env of `fA()`
#<environment: R_GlobalEnv>  #<- parent is `eval`ed as parent of `fA()`'s current

As Taz notes, the distinction between default and supplied arguments is stated in the definition manual. Fooling a bit around we could see it in action by trying to reach the promises of arguments:

First, a helper function to access the promises of current arguments:

.ff = inline::cfunction(sig = c(symarg = "symbol", env = "environment", penv = "environment"), body = '
    SEXP arg = findVar(symarg, env), ans = allocVector(VECSXP, 5);
    SET_VECTOR_ELT(ans, 0, PRCODE(arg)); 
    SET_VECTOR_ELT(ans, 1, PRENV(arg));
    SET_VECTOR_ELT(ans, 2, eval(PRCODE(arg), PRENV(arg)));
    SET_VECTOR_ELT(ans, 3, env);
    SET_VECTOR_ELT(ans, 4, penv);
    return(ans);
')

And the function whose arguments we 'll track:

ff = function(arg = parent.frame())
{
    ans = setNames(.ff(quote(arg), environment(), parent.frame()), 
                   c("expr", "envir", "val", "cur", "par"))

    cat(sprintf("promise:\n\tcall: '%s'\n\tsearched at: '%s'\n\tfound as: '%s'\ncurrent: '%s'\nparent: '%s'\n%s\n", 
                deparse(ans$expr), capture.output(ans$envir), 
                capture.output(ans$val), capture.output(ans$cur),
                capture.output(ans$par), strrep("-", 40)))

    return(invisible(ans))
}

And a simple example:

ff()
#promise:
#        call: 'parent.frame()'
#        searched at: '<environment: 0x06fff594>'
#        found as: '<environment: R_GlobalEnv>'
#current: '<environment: 0x06fff594>'
#parent: '<environment: R_GlobalEnv>'
#----------------------------------------

ff(parent.frame())
#promise:
#        call: 'parent.frame()'
#        searched at: '<environment: R_GlobalEnv>'
#        found as: '<environment: R_GlobalEnv>'
#current: '<environment: 0x06fcce20>'
#parent: '<environment: R_GlobalEnv>'
#----------------------------------------

Or a more nested case:

fnest1 = function() ff()
fnest2 = function() ff(parent.frame())

fnest1()
#promise:
#        call: 'parent.frame()'
#        searched at: '<environment: 0x028d1ccc>'
#        found as: '<environment: 0x028d1d20>'
#current: '<environment: 0x028d1ccc>'
#parent: '<environment: 0x028d1d20>'
#----------------------------------------

fnest2()
#promise:
#        call: 'parent.frame()'
#        searched at: '<environment: 0x026866b8>'
#        found as: '<environment: R_GlobalEnv>'
#current: '<environment: 0x0268662c>'
#parent: '<environment: 0x026866b8>'
#----------------------------------------

Problem

Original source

Related problems

• When/how/where is parent.frame in a default argument interpreted?