从索引中获取列表元素名称

Question

我正在尝试使用以下 glue 代码来创建信息性错误消息

library(rlang)
library(glue)

my_function <- function(x) {
  UseMethod("my_function", x)
}

my_function.default <- function(x) {
  abort(glue(
    "Can't calculate my_function because { deparse(substitute(x)) } is of type ",
    glue_collapse(class(x))
  ))
}

使用这个测试列表我们看到它有效：

test <- list(
  x = c(1,2,3),
  y = c("one", "two", "three")
)

my_function(test[[1]])

Error: Can't calculate my_function because test[[1]] is of type numeric
Run `rlang::last_error()` to see where the error occurred.

但是是否可以使用 glue 来获得错误 return x，其中显示 test[[1]] 导致错误：

Can't calculate my_function because x is of type numeric

Answer 1

这是一个深入索引表达式以推断被索引元素名称的函数。简而言之，它将遵循 list[index] 模式的表达式转换为 names(list)[index]，同时智能地处理 list$name 已经在表达式中具有名称。

getElementNames <- function(ee) {
    ## Determine if ee is an indexing operation
    eel <- as.list(ee)
    isIdx <- purrr::map_lgl(exprs( `[`, `[[`, `$` ),
                            identical, eel[[1]])

    ## If not, return the expression itself as a string
    if(!any(isIdx)) return( deparse(ee) )

    ## The name may already be in the expression
    if( is.name(eel[[3]]) || is.character(eel[[3]]) )
        return( as.character(eel[[3]]) )

    ## Compose an expression indexing the names
    nms <- eval.parent(expr( names(!!eel[[2]])[!!eel[[3]]] ))

    ## Names might be missing
    `if`( is.null(nms), deparse(ee), nms )
}

实际函数：

test  <- list(a=4, b=5, c=6)
test2 <- 1:3
ftest <- function(x) abort(glue("Can't calculate {getElementNames(substitute(x))}"))

ftest( test[[2]] )    # index by numeric value
# Error: Can't calculate b

ftest( test$c )       # index by name
# Error: Can't calculate c

ftest( test[["a"]] )  # another way to index by name
# Error: Can't calculate a

i <- 2; j <- 3
ftest( test[i:j] )    # index multiple elements
# Error: Can't calculate b
# * Can't calculate c

ftest( test2[3] )     # index something with no names
# Error: Can't calculate test2[3]

ftest( fun_that_returns_list() )  # non-indexing expression
# Error: Can't calculate fun_that_returns_list()

ftest( 1:3 )                      # another non-indexing expression
# Error: Can't calculate 1:3

从索引中获取列表元素名称

Get list element name from index

r

rlang

r-glue