confusing behavior of purrr::pmap with rlang; "to quote" or not to quote argument that is the Q

Question

I have a custom function where I am reading entered variables from a dataframe using rlang. This function works just fine irrespective of whether the arguments entered are quoted or unquoted. But, strangely enough, when this function is used with purrr::pmap, it works only if the argument is quoted.

So I have two questions:

1. Why does the function behavior this way?

2. How can I make a function using rlang such that I won't have to quote the arguments even if used in purrr::pmap?

Here is a minimal reprex that uses a simple function to highlight this issue:

# loading the needed libraries
library(rlang)
library(dplyr)
library(purrr)


# defining the function
tryfn <- function(data, x, y) {
  data <-
    dplyr::select(
      .data = data,
      x = !!rlang::enquo(x),
      y = !!rlang::enquo(y)
    )

  # creating a dataframe of means
  result_df <- data.frame(mean.x = mean(data$x), mean.y = mean(data$y))

  # return the dataframe
  return(result_df)
}

# without quotes (works!)
tryfn(iris, Sepal.Length, Sepal.Width)
#>     mean.x   mean.y
#> 1 5.843333 3.057333

# with quotes (works!)
tryfn(iris, "Sepal.Length", "Sepal.Width")
#>     mean.x   mean.y
#> 1 5.843333 3.057333

# pmap without quotes (doesn't work)
purrr::pmap(.l = list(
  data = list(iris, mtcars, ToothGrowth),
  x = list(Sepal.Length, wt, len),
  y = list(Sepal.Width, mpg, dose)
),
.f = tryfn)
#> Error in is.data.frame(.l): object 'Sepal.Length' not found

# pmap with quotes (works!)
purrr::pmap(.l = list(
  data = list(iris, mtcars, ToothGrowth),
  x = list("Sepal.Length", "wt", "len"),
  y = list("Sepal.Width", "mpg", "dose")
),
.f = tryfn)
#> [[1]]
#>     mean.x   mean.y
#> 1 5.843333 3.057333
#> 
#> [[2]]
#>    mean.x   mean.y
#> 1 3.21725 20.09062
#> 
#> [[3]]
#>     mean.x   mean.y
#> 1 18.81333 1.166667

Created on 2018-05-21 by the reprex package (v0.2.0).

Answer 1

The problem was: R saw Sepal.Length, wt, len symbols so it tried to look in the current environment and evaluated them. Of course it resulted in errors as they were columns of a data frame. When you quoted them, R didn't try to evaluate and returned values as it saw those as strings.

If you replace list with base::alist or dplyr::vars or rlang::exprs, it should work

Note: as we already quote the inputs, we don't need to use rlang::enquo inside tryfn anymore.

# loading the needed libraries
library(rlang)
library(tidyverse)

# defining the function
tryfn <- function(data, x, y) {
  data <-
    dplyr::select(
      .data = data,
      x = !! x,
      y = !! y
    )

  # creating a data frame of means
  result_df <- data.frame(mean.x = mean(data$x), mean.y = mean(data$y))

  # return the data frame
  return(result_df)
}

# alist handles its arguments as if they described function arguments. 
# So the values are not evaluated, and tagged arguments with no value are 
# allowed whereas list simply ignores them. 

purrr::pmap(.l = list(
  data = list(iris, mtcars, ToothGrowth),
  x    = alist(Sepal.Length, wt, len),
  y    = alist(Sepal.Width, mpg, dose)
),
.f = tryfn)

#> [[1]]
#>     mean.x   mean.y
#> 1 5.843333 3.057333
#> 
#> [[2]]
#>    mean.x   mean.y
#> 1 3.21725 20.09062
#> 
#> [[3]]
#>     mean.x   mean.y
#> 1 18.81333 1.166667


purrr::pmap(.l = list(
  data = list(iris, mtcars, ToothGrowth),
  x    = dplyr::vars(Sepal.Length, wt, len),
  y    = dplyr::vars(Sepal.Width, mpg, dose)
),
.f = tryfn)

#> [[1]]
#>     mean.x   mean.y
#> 1 5.843333 3.057333
#> 
#> [[2]]
#>    mean.x   mean.y
#> 1 3.21725 20.09062
#> 
#> [[3]]
#>     mean.x   mean.y
#> 1 18.81333 1.166667

purrr::pmap(.l = list(
  data = list(iris, mtcars, ToothGrowth),
  x    = rlang::exprs(Sepal.Length, wt, len),
  y    = rlang::exprs(Sepal.Width, mpg, dose)
),
.f = tryfn)

#> [[1]]
#>     mean.x   mean.y
#> 1 5.843333 3.057333
#> 
#> [[2]]
#>    mean.x   mean.y
#> 1 3.21725 20.09062
#> 
#> [[3]]
#>     mean.x   mean.y
#> 1 18.81333 1.166667

Created on 2018-05-21 by the reprex package (v0.2.0).