Odd error when combining `nested_cv` and `sliding_period`

[pietrofranceschi]

The problem

I'm having trouble when combining sliding_period with nested_cv, in particular if I specify the origin argument together with lookback (even with its default value).

The error is the following

Error in if (grepl("^bootstraps", deparse(outer_cl))) { : 
  the condition has length > 1

Here a reproducible example:

library(tidyverse)
library(rsample)
library(modeldata)

data(Chicago)


## sliding period alone: it works fine!!
outside <-  sliding_period(Chicago, 
                           index = date, 
                           period = "month", 
                           lookback = 0, 
                           origin = Chicago$date[1])


## Now the same in a nested structure: it works fine!
test_1 <- nested_cv(Chicago, 
                   outside = sliding_period(index = date, 
                                            period = "month", 
                                            origin = Chicago$date[1]),  ## here I'm only using the origin
                   inside = vfold_cv(v = 4)
)


## Now the same structure adding the loopback argument: Error!
test_2 <- nested_cv(Chicago, 
                    outside = sliding_period(index = date, 
                                             period = "month", 
                                             origin = Chicago$date[1],
                                             lookback = 0),
                    inside = vfold_cv(v = 4)
)
#> Error in if (grepl("^bootstraps", deparse(outer_cl))) {: the condition has length > 1


## And finally with the loopback but without the origin and with the lookback: it works fine!
test_3 <- nested_cv(Chicago, 
                    outside = sliding_period(index = date, 
                                             period = "month", 
                                             lookback = 0),
                    inside = vfold_cv(v = 4)
)
Created on 2023-11-02 with [reprex v2.0.2](https://reprex.tidyverse.org/)

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[mikemahoney218]

This is a really funny bug -- the issue is that nested_cv() is checking to see if you're passing bootstraps() to outside, which will trigger a warning. To check that, it deparses the value of outside using deparse(), which returns a character vector, then uses grepl() to see if bootstraps is anywhere in the actual text of the arguments you passed.

The issue is that deparse() will break long calls into multiple elements, which causes the grepl() call to return a vector of multiple TRUE/FALSE values, which breaks the if() statement. So the triggering issue is that your argument to outside is too long. You can actually sneak around this by using partial argument matching, which gets the call to outside under the limit:

library(tidyverse)
library(rsample)
library(modeldata)

data(Chicago)
nested_cv(Chicago, 
          outside = sliding_period(i = date, 
                                   p = "month", 
                                   origin = Chicago$date[1]),
          inside = vfold_cv(v = 4)
)
#> # Nested resampling:
#> #  outer: Sliding period resampling
#> #  inner: 4-fold cross-validation
#> # A tibble: 187 × 3
#>    splits          id       inner_resamples
#>    <list>          <chr>    <list>         
#>  1 <split [10/28]> Slice001 <vfold [4 × 2]>
#>  2 <split [28/31]> Slice002 <vfold [4 × 2]>
#>  3 <split [31/30]> Slice003 <vfold [4 × 2]>
#>  4 <split [30/31]> Slice004 <vfold [4 × 2]>
#>  5 <split [31/30]> Slice005 <vfold [4 × 2]>
#>  6 <split [30/31]> Slice006 <vfold [4 × 2]>
#>  7 <split [31/31]> Slice007 <vfold [4 × 2]>
#>  8 <split [31/30]> Slice008 <vfold [4 × 2]>
#>  9 <split [30/31]> Slice009 <vfold [4 × 2]>
#> 10 <split [31/30]> Slice010 <vfold [4 × 2]>
#> # ℹ 177 more rows

^{Created on 2023-11-02 with reprex v2.0.2}

To be clear, this is a bug and I opened #461 to fix it. I just think it's very funny that this bug winds up being "the argument to outside used too many characters". Who would have thought that was a thing that could happen?

Thanks for the excellent, excellent reprex -- made it so I could immediately see what was wrong, and I've stolen your example for #461 as a test to make sure this gets & stays fixed.

[hfrick]

To echo Mike here: thanks a lot for the bug report with the excellent reprex! The fix is merged into the dev version 👍

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