如何使用 purrr::map（未迭代）附加 arguments？

Question

I thought I had understood how to use the additional arguments argument ( ... ) of purrr::map . Here is some code that hopefully illustrates the (to me) unexpected behaviour of purrr::map :

It seems that passing argument a as additional argument in purrr::map is not working:

library(purrr)

f <- function(a, b) {
  a + b
}

g <- function(a = 0, b) {
  a + b
}

map(1:3, .f = ~ f(b = .x, a = 1))
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 4
map(1:3, .f = ~ f(b = .x), a = 1)
#> Error in f(b = .x): argument "a" is missing, with no default

map(1:3, .f = ~ g(b = .x, a = 1))
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 4
map(1:3, .f = ~ g(b = .x), a = 1)
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 2
#> 
#> [[3]]
#> [1] 3

lapply(1:3, function(b, a = 1) f(a, b))
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 4
lapply(1:3, function(b, a) f(a, b), a = 1)
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 4

My question is why does the code:

map(1:3, .f = ~ f(b = .x), a = 1)

throw an error?

Answer 1

We could pass the remaining arguments without any anonymous function

library(purrr)
map(1:3, f, a = 1)
#[[1]]
#[1] 2

#[[2]]
#[1] 3

#[[3]]
#[1] 4

---

Or another option is rlang::as_function or purrr:as_mapper

map(1:3, as_mapper(f), a = 1)

Or create the f on the fly

map(1:3, as_mapper(~ .x + .y), a = 1)

---

Or call it in invoke

map(1:3, ~ invoke(f, b = .x, a = 1))
#[[1]]
#[1] 2

#[[2]]
#[1] 3

#[[3]]
#[1] 4

This would make it more easier to read than the .f = ~ f(b =.x), a = 1

Answer 2

Behind the scenes, map() calls as_mapper() . We can do this by hand to see what's going on:

purrr::as_mapper( ~ f(b = .x, a = 1) )
# <lambda>
# function (..., .x = ..1, .y = ..2, . = ..1) 
# f(b = .x, a = 1)                                <----
# attr(,"class")
# [1] "rlang_lambda_function" "function"


purrr::as_mapper( ~ f(b = .x), a=1 )
# <lambda>
# function (..., .x = ..1, .y = ..2, . = ..1) 
# f(b = .x)                                       <----
# attr(,"class")
# [1] "rlang_lambda_function" "function"           

I highlighted the important distinction with <--- . Notice that in the second case, the lambda function that gets created does not incorporate your extra a=1 parameter, which leads to the error you are observing.

To address your comment, a=1 actually is being passed to the lambda function. Your lambda function just isn't doing anything with it. To properly incorporate a , the lambda function definition needs to handle the ... dots:

g <- function(a, b, ...) {a + b}               # ... are needed to catch all extra 
                                               #   arguments from as_mapper

purrr::as_mapper( .f = ~ g(b=.x, ...) )
# <lambda>
# function (..., .x = ..1, .y = ..2, . = ..1) 
# g(b = .x, ...)                               <-- dots are now forwarded to g()
# attr(,"class")
# [1] "rlang_lambda_function" "function"            

purrr::map(1:3, .f = ~ g(b=.x, ...), a=1 )     # a now properly gets passed to g
# [[1]]
# [1] 2
#
# [[2]]
# [1] 3
#
# [[3]]
# [1] 4