强大的测试是否可以将表达式强制转换为字符以避免 as.character 错误

Question

I am looking for a reliable test (= does not throw an error) whether an expression can be converted to character.

How could I do this?

I want to avoid that calling as.character throws an error like eg. in case of as.character(function() 1) which says:

Error in as.character(function() 1) : 
  cannot coerce type 'closure' to vector of type 'character'

Example expressions:

# do not throw an error in as.character
1
1:10
"test"
NA
1 + 2
# do throw an error in as.character
function() 1
new.env()

PS: Of course I could use tryCatch but is there an easier way?

Edit 1: I want to test if the result value of an expression can be coerced to a character (not the expression itself)

Answer 1

I'm sure there will be better answers to this question and I look forward to reading them. Wanting evaluated results assumes they are prone to evaluation, but if any unknown input is accepted, one can't be sure.

Depending upon where your pipeline is going next, explore variants of quote or expression .

as.character(quote(function() 1))
[1] "function"                      "NULL"                         
[3] "1"                             "c(1, 20, 1, 31, 20, 31, 1, 1)"

as.character(expression(1 + 2))
[1] "1 + 2"

as.character(eval(expression(1+2)) # works in this case
[1] "3"

as.character(eval(expression(1+2, 'A') # what would this be?
[1] "A" # so nonsense

Another approach might be to find out what sort of thing in the wild you're being passed and then decide what to do with it:

as.character(typeof(function() 1))
[1] "closure" # not safe or just pass along "closure" and move on

as.character(typeof(new.env())
[1] "environment" # not safe

as.character(typeof(expression(1+2))
[1] "expression" # safe (maybe) for further manipulation, replace typeof with eval

But typeof isn't immune to other user errors on the way to evaluated results

as.character(typeof(function(x) 1, 'a'))
Error in typeof(function(x) 1, "a") : unused argument ("a")

I find this an interesting question because I've always assumed there was a way to render to string any object in R, though the path may be convoluted.

You'll know best regards your full project and where you are trying to control evaluation.