varying_args() / find_varying() should detect lists of length 0

[DavisVaughan]

Below is find_varying(). There is a point where it calls map_lgl over a potential list, but if this list is empty, it will return logical(0). This just happens to work because we then call any(logical(0)) which returns FALSE (which is what we want), but I'd rather it actually return the right thing. I think we can just say the following in the marked spot below

if (length(x) > 0) {
   map_lgl(x, find_varying)
}
FALSE

function (x) 
{
    if (is_quosure(x)) 
        x <- quo_get_expr(x)
    if (is_varying(x)) {
        return(TRUE)
    }
    else if (is.atomic(x) | is.name(x)) {
        FALSE
    }
    else if (is.call(x) || is.pairlist(x)) {
        for (i in seq_along(x)) {
            if (is_varying(x[[i]])) 
                return(TRUE)
        }
        FALSE
    }
    else if (is.vector(x) | is.list(x)) {
        map_lgl(x, find_varying) # <- RIGHT HERE
    }
    else {
        stop("Don't know how to handle type ", typeof(x), call. = FALSE)
    }
}

if you want to see it happening, you can use this code

library(parsnip)
library(recipes)

x <- recipe(Species ~ ., data = iris) %>%
  step_bs(Sepal.Length, degree = varying())

# debugonce(varying_args)
varying_args(x$steps[[1]])

after running debugonce() and then calling the varying_args() call, step through until you get to the res <- map(x, find_varying) line, where res will hold logical(0) for options

[DavisVaughan]

minimal example:

parsnip:::find_varying(list())
#> logical(0)

^{Created on 2019-01-25 by the reprex package (v0.2.1.9000)}

[DavisVaughan]

for the sake of the algorithm, it also makes sense for find_varying() to always return a single logical value. Currently, in the case of lists, it returns multiple logicals. This is handled in varying_args() methods by a call to any(), but it should happen at the find_varying() level for consistency in return value.

parsnip:::find_varying(list(1, parsnip::varying()))
#> [1] FALSE  TRUE

^{Created on 2019-01-25 by the reprex package (v0.2.1.9000)}

[github-actions]

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