Add omitted argument name to 19.7.5 exercise 1

[ryan-heslin]

Working through the chapter 19 exercises, I encountered an error doing the first exercise of section 19.7.5. I tried replacing the last line of the provided function linear with reduce(summands, call2, "+"), as suggested. Running the code below triggered an error:

library(rlang)
library(purrr)

linear <- function(var, val) {
  var <- ensym(var)
  coef_name <- map(seq_along(val[-1]), ~ expr((!!var)[[!!.x]]))

  summands <- map2(val[-1], coef_name, ~ expr((!!.x * !!.y)))
  summands <- c(val[[1]], summands)

  reduce(summands, call2, "+")
}

linear(x, c(10, 5, -4))

Attaching package: ‘purrr’

The following objects are masked from ‘package:rlang’:

    %@%, as_function, flatten, flatten_chr, flatten_dbl, flatten_int,
    flatten_lgl, flatten_raw, invoke, splice

Error in `fn()`:
! Can't create call to non-callable object
Backtrace:
    ▆
 1. ├─global linear(x, c(10, 5, -4))
 2. │ └─purrr::reduce(summands, call2, "+")
 3. │   └─purrr:::reduce_impl(.x, .f, ..., .init = .init, .dir = .dir)
 4. │     └─rlang fn(out, elt, ...)
 5. └─rlang::abort(message = message)

It seems that reduce passes one element of summands to call2's .fn argument, and the other element and "+" to its .... Changing the line to reduce(summands, call2, .fn = "+") so the argument is matched correctly fixes the issue, and the code returns the correct result. This pull request changes the suggested line in the exercise accordingly. Since the question is about code readability rather than argument matching, I reasoned the behavior was unintended.

My sessionInfo() output:

> sessionInfo()
R version 4.2.0 (2022-04-22)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.4 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rlang_1.0.1 purrr_0.3.4

loaded via a namespace (and not attached):
[1] compiler_4.2.0 magrittr_2.0.2 cli_3.1.1 

I assign the copyright of this contribution to Hadley Wickham.