I have been exploring the various application of using `pmap` function and its variations recently and I am particularly interested in using `c(...)` to pass all the arguments into. The following data set belongs to another question that we discussed [earlier today](https://stackoverflow.com/questions/67046753/r-repeat-in-column-based-on-value-in-row) with a number of very knowledgeable users.
We were supposed to repeat the values in `weight` column based on values in `Days` column along their respective rows to get the following output:

```
df <- tribble(
  ~Name,    ~School,   ~Weight, ~Days,
  "Antoine", "Bach",     0.03,   5,
  "Antoine", "Ken",      0.02,   7,
  "Barbara", "Franklin", 0.04,   3
)

```

Output:

```
df %>%
  mutate(map2_dfr(Weight, Days, ~ set_names(rep(.x, .y), 1:.y))) %>%
  select(-c(Weight, Days))

# A tibble: 3 x 9
  Name    School     `1`   `2`   `3`   `4`   `5`   `6`   `7`
  <chr>   <chr>    <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 Antoine Bach      0.03  0.03  0.03  0.03  0.03 NA    NA   
2 Antoine Ken       0.02  0.02  0.02  0.02  0.02  0.02  0.02
3 Barbara Franklin  0.04  0.04  0.04 NA    NA    NA    NA 

```

My question is this output is achievable through various solutions but the following one proposed by one of the contributors caught my attention. I would like to know how I could rewrite it by means of `c(...)`

```
# This is not my code and it works:

pmap_dfr(df, function(Weight, Days, ...) c(..., setNames(rep(Weight, Days), 1:Days)))

# And I can also rewrite it in the following way which also works:

df %>%
  mutate(data = pmap(list(Weight, Days), ~ setNames(rep(.x, .y), 1:.y))) %>%
  unnest_wider(data)

```

But I would like to know why any of these doesn't work:

```
df %>% 
  mutate(pmap_dfr(., ~ c(..., setNames(rep(Weight, Days), 1:Days))))

df %>% 
  pmap_dfr(., ~ c(..., setNames(rep(Weight, Days), 1:Days)))

```

Thank you very much in advance and so sorry for the long description.