Getting an R expression from a value (similar to enquote)

Question

Assume I have a value x which is of some (unknown) type (especially: scalar, vector or list). I would like to get the R expression representing this value. If x == 1 then this function should simply return expression(1). For x == c(1,2)) this function should return expression(c(1,2)). The enquote function is quite near to that what I want, but not exactly.

By some playing around I found the following "solution" to my problem:

get_expr <- function(val) {
  tmp_expr <- enquote(val)
  tmp_expr[1] <- quote(expression())
  return(eval(tmp_expr))
}

get_expr(1) # returns expression(1)
get_expr(c(1, 2)) # returns expression(c(1, 2))
get_expr(list(x = 1)) # returns expression(list(x = 1))

But I think my get_expr function is some kind of hack. Logically, the evaluation should not be necessary.

Is there some more elegant way to do this? As far as I see, substitute does not really work for me, because the parameter of my get_expr function may be the result of an evaluation (and substitute(eval(expr)) does not do the evaluation).

I found another way via parse(text = deparse(val)), but this is even more a bad hack...

Answer 1

as.expression(list(...)) seems to do it:

> get_expr <- function(val) as.expression(list(val))
> str(get_expr(1))
  expression(1)
> str(get_expr(c(1, 2)))
  expression(c(1, 2))
> str(get_expr(list(x=1)))
  expression(list(x = 1))
> val <- list(x=1, y=2)
> str(get_expr(val))
  expression(list(x = 1, y = 2))

Answer 2

You can use substitute(), and just need to call it a bit differently:

express <- function(e) substitute(expression(x), env = list(x=e))

v1 <- c(1, 2)
express(v1)
# expression(c(1, 2))

v2 <- list(a = 1, b = 2)
express(v2)
# expression(list(a = 1, b = 2))