Constructing lists using tidyeval tools (like `!!`

问题:

I am looking a way for easy list constructing based on R's tidyeval framework as defined in the rlang package.

Below is what I want to achieve:

a <- "item_name"
b <- "item_value"
identical(
    list(!!a := !!b),    # list(!!a := b) is of course also fine
    list(item_name = "item_value")
)

What I can obtain at the moment is:

list(!!a := !!b)
# output
[[1]]
`:=`(!(!a), !(!b)

Alternatively it can get perhaps a little bit better when adding quosure:

quo(list(!!a := !!b))
# output
<quosure: global>
~list(`:=`("item_name", "item_value"))

Unfortunately I have no idea how to proceed further from here.

In other words I would like to have a similar effect like what we can get in the dplyr package:

transmute(iris, !!a := b)
# first few rows
    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species  item_name
1            5.1         3.5          1.4         0.2     setosa item_value
2            4.9         3.0          1.4         0.2     setosa item_value
3            4.7         3.2          1.3         0.2     setosa item_value
4            4.6         3.1          1.5         0.2     setosa item_value
5            5.0         3.6          1.4         0.2     setosa item_value
6            5.4         3.9          1.7         0.4     setosa item_value

回答1:

You can use rlang::list2() which supports name-unquoting with := and splicing with !!!.

Note that you shouldn't unquote the argument itself since list2() is not a quoting function, it is just like list() with a few more syntactic features:

a <- "item_name"
b <- "item_value"

list2(!!a := b)