It looks like it's not that the `train`

is named-list-column, but that the larger tibble containining it has a *-list column:

```
library(tidyverse)
library(rsample)
iris %>%
vfold_cv() %>%
mutate(train = map(splits, training))
#> # 10-fold cross-validation
#> # A tibble: 10 x 3
#> splits id train
#> * <named list> <chr> <named list>
#> 1 <split [135/15]> Fold01 <df[,5] [135 × 5]>
#> 2 <split [135/15]> Fold02 <df[,5] [135 × 5]>
#> 3 <split [135/15]> Fold03 <df[,5] [135 × 5]>
#> 4 <split [135/15]> Fold04 <df[,5] [135 × 5]>
#> 5 <split [135/15]> Fold05 <df[,5] [135 × 5]>
#> 6 <split [135/15]> Fold06 <df[,5] [135 × 5]>
#> 7 <split [135/15]> Fold07 <df[,5] [135 × 5]>
#> 8 <split [135/15]> Fold08 <df[,5] [135 × 5]>
#> 9 <split [135/15]> Fold09 <df[,5] [135 × 5]>
#> 10 <split [135/15]> Fold10 <df[,5] [135 × 5]>
iris %>%
vfold_cv() %>%
mutate(train = map(splits, training)) %>%
select(-1)
#> # A tibble: 10 x 2
#> id train
#> <chr> <named list>
#> 1 Fold01 <df[,5] [135 × 5]>
#> 2 Fold02 <df[,5] [135 × 5]>
#> 3 Fold03 <df[,5] [135 × 5]>
#> 4 Fold04 <df[,5] [135 × 5]>
#> 5 Fold05 <df[,5] [135 × 5]>
#> 6 Fold06 <df[,5] [135 × 5]>
#> 7 Fold07 <df[,5] [135 × 5]>
#> 8 Fold08 <df[,5] [135 × 5]>
#> 9 Fold09 <df[,5] [135 × 5]>
#> 10 Fold10 <df[,5] [135 × 5]>
iris %>%
vfold_cv() %>%
mutate(train = map(splits, training)) %>%
select(-1) %>%
hoist(train, Sepal.Length = 'Sepal.Length')
#> # A tibble: 10 x 3
#> id Sepal.Length train
#> <chr> <named list> <named list>
#> 1 Fold01 <dbl [135]> <df[,4] [135 × 4]>
#> 2 Fold02 <dbl [135]> <df[,4] [135 × 4]>
#> 3 Fold03 <dbl [135]> <df[,4] [135 × 4]>
#> 4 Fold04 <dbl [135]> <df[,4] [135 × 4]>
#> 5 Fold05 <dbl [135]> <df[,4] [135 × 4]>
#> 6 Fold06 <dbl [135]> <df[,4] [135 × 4]>
#> 7 Fold07 <dbl [135]> <df[,4] [135 × 4]>
#> 8 Fold08 <dbl [135]> <df[,4] [135 × 4]>
#> 9 Fold09 <dbl [135]> <df[,4] [135 × 4]>
#> 10 Fold10 <dbl [135]> <df[,4] [135 × 4]>
```

^{Created on 2020-03-11 by the reprex package (v0.3.0)}