[Rd] [External] Warning with new placeholder piped to data.frame extractors `[` and `[[`.

[Rui Barradas]

Hello,

I agree with several points you've made.

The code of the data.frame methods for `[` and `[[` is already 
complicated enough and a revision is probably not worth the effort, 
constructs like piping to `[` and `[[` is not furthering the cause of 
readability and a new base R dplyr::pull like function would put an 
extra development and maintenace burden on the R Core Team, to which we 
are in great debt for their excellent and already difficult and time 
consuming work developing, maintaining and making R evolve along the years.

My question, if the named argument syntax is mandatory then it should 
not throw a warning, seems to have raised a consensus that this use of 
the new pipe operator and placeholder should be discouraged (Toby), 
considered a bug (Gabriel) or maybe intentional (Duncan). Definitely an 
unclear idiom to be avoided and not a priority.

I still find it strange but if R is telling the programmer to write 
better code then follow the advice.

(As a side note, all of the following work as expected:

1:6 |> `[`(x = _, 2)
1:6 |> `[[`(x = _, 2)

matrix(1:6, nrow = 3) |> `[`(x = _, 2, 2)
matrix(1:6, nrow = 3) |> `[`(x = _, 2, )
matrix(1:6, nrow = 3) |> `[`(x = _, , 2)

list(1:6, b = 7:10) |> `[`(x = _, 2)
list(1:6, b = 7:10) |> `[[`(x = _, 2)
list(1:6, b = 7:10) |> `$`(x = _, 'b')

So this is specific to the data.frame methods.)

Hope this helps,

Rui Barradas

Às 23:44 de 18/07/2022, luke-tierney using uiowa.edu escreveu:
> On Sat, 16 Jul 2022, Rui Barradas wrote:
> 
>> Hello,
>>
>> When piping to any of `[.data.frame` or `[[.data.frame`, the 
>> placeholder in mandatory.
>>
>>
>> df1 <- data.frame(y = 1:10, f = rep(c("a", "b"), each = 5))
>>
>> aggregate(y ~ f, df1, mean) |> `[`('y')
>> # Error: function '[' not supported in RHS call of a pipe
>>
>> aggregate(y ~ f, df1, mean) |> `[[`('y')
>> # Error: function '[' not supported in RHS call of a pipe
>>
>>
>>
>> But if used it throws a warning.
>>
>>
>>
>> aggregate(y ~ f, df1, mean) |> `[`(x = _, 'y')
>> #  Warning in `[.data.frame`(x = aggregate(y ~ f, df1, mean), "y"): 
>> named arguments
>> #  other than 'drop' are discouraged
>> #    y
>> #  1 3
>> #  2 8
>>
>> aggregate(y ~ f, df1, mean) |> `[[`(x = _, 'y')
>> #  Warning in `[[.data.frame`(x = aggregate(y ~ f, df1, mean), "y"): 
>> named
>> #  arguments other than 'exact' are discouraged
>> #  [1] 3 8
>>
> 
> The pipe syntax requirs that the placeolder be used as a named
> argument.  If you do that, then the syntax is legal and parses
> successfully.
> 
>> Hasn't this become inconsistent behavior?
>> More than merely right, the named argument is mandatory, it shouldn't 
>> give warnings.
> 
> Any R function can decide whether it wants to allow explicitly named
> arguments.  Disallowing or discouraging using explicitly named
> arguments requires some work and is usually not a good idea. In the
> case of the data.frame mechods for [ and [[ the decision was made to
> discourage using named arguments other than 'exact'. This seems to
> have been to allow a more an expedient way to implement these
> functions. This could be revisited, but I doubt is is worth the effort.
> 
> For me the main reason for using pipes is to make code more
> readable. Using `[` and such constructs is not furthering that
> cause. When I use pipes I am almost always using tidyverse
> features, so I have dpyr::pull available, which is more readable,
> to me at least. Arguably, base R could have a similar function,
> but again I doubt this would be a good investment of time.
> 
> An option that we have experimented with is to allow the placeholder
> at the head of an extraction chain. This is supported in the
> experimental branch at
> https://svn.r-project.org/R/branches/R-syntax. So for example:
> 
>      > mtcars |> _$cyl[1]
>      [1] 6
> 
> This may make it into R-devel for the next release, but it still needs
> more testing.
> 
> Best,
> 
> luke
> 
>>
>> Hope this helps,
>>
>> Rui Barradas
>>
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