Confusing error message

[jennybc]

If you forget to specify the col which unnest_wider() should act on, it complains that x is missing. Feels like that should be col. It's not entirely clear to me if this is a tidyr or tidyselect problem

library(tidyverse)

df <- tibble(
  z = list(list(letter = "a"), list(letter = "b"))
)

df %>% 
  unnest_wider(z)
#> # A tibble: 2 x 1
#>   letter
#>   <chr> 
#> 1 a     
#> 2 b

df %>% 
  unnest_wider()
#> Error: argument "x" is missing, with no default

^{Created on 2019-11-19 by the reprex package (v0.3.0.9001)}

cc @lionel-

[lionel-]

I think we should compact missing arguments the same way we compact NULL.

tidyselect::eval_select(quote(c(NULL, NULL)), iris)
#> named integer(0)

tidyselect::eval_select(quote(c(, )), iris)
#> Error: argument "x" is missing, with no default

But also tidyr should either give a default value of NULL to signal the argument is optional, or explicitly check that it is empty.

[jennybc]

I've been poking around other tidyr functions and many of them display the same problem (unpack(), other rectangling functions). Basically whenever a missing, e.g., .col is immediately handled viaenquo(.col), it prevents the standard message 'argument ".col" is missing, with no default' and, by the time it's emitted, the complaint is about "x".

[hadley]

@lionel- the code is currently:

col <- tidyselect::vars_select(tbl_vars(data), !!enquo(col))

which will become:

col <- tidyselect::vars_select(tbl_vars(data), {{ col }})

So it seems to me that it should be tidyselect's (or {{'s?) responsibility to trigger the correct error

[lionel-]

{{ cannot give an error with missing arguments because it is valid to defuse them (and they can be inspected with quo_is_missing()).

I was thinking tidyselect would not treat it as an error, but as an empty selection. This makes NULL and missing arguments consistent. Then it makes sense for tidyr to make these explicitly optional by taking a default of NULL. If tidyr prefers to keep this an error, it'd have to check that the argument is missing.

[hadley]

@lionel- so you think I should be doing if (missing(col)) abort("Argument col is missing with no default")? That seems like a weird gotcha.

[hadley]

Can you help me understand why it's different to this?

library(rlang)

x <- expr()
expr(!!x)
#> Error in enexpr(expr): argument "x" is missing, with no default

^{Created on 2019-11-26 by the reprex package (v0.3.0)}

[lionel-]

In your example the missing argument is not wrapped in a quosure. Normally it would be wrapped via enquo() or {{.

x <- quo()
expr(!!x)
#> <quosure>
#> expr: ^
#> env:  empty

[hadley]

Oh duh, yeah, still seems weird that I need to manually check that col exists.