Check "emptiness" of list containing empty vectors (which R does not recognise as empty list)

I have a list that is the result of a row selection in a data frame. The issue is that sometimes there is no row to select and it returns a list in this form: a non-empty list with no actual content.

L <- list(combattech = character(0), damage = character(0), bonus = character(0), 
          range = structure(list(close = character(0), medium = character(0), far = character(0)), 
                            row.names = integer(0), class = "data.frame"), 
          ammo = character(0), weight = character(0), name = character(0), 
          price = character(0), sf = character(0))

I want to verify if I actually have a meaningful result and not a list with all elements being empty vectors. But a list with empty vectors is not equivalent to an empty list:

length(L) == 0
#> [1] FALSE

does not give me TRUE because the length is 9 not 0.

Of course, I could simply check if length( which(...row selection...) ) before I pick the selection and usually I do, but in this case I do not have access to the original row indices.

all(sapply(L, length) == 0)
#> [1] FALSE

also does not work (i.e. returns FALSE) because the nested data structure range returns 3.

^{Created on 2020-06-28 by the reprex package (v0.3.0)}

Solution

1) We can use rapply to recursively walk the structure and return a flat result.

all(rapply(L, length) == 0)
## [1] TRUE

2) Another approach is to unlist it first:

length(unlist(L)) == 0
## [1] TRUE