r - invisible() 返回的对象何时不再不可见？

Question

?invisible说

Return a (temporarily) invisible copy of an object.

那个括号暗示隐形不会永远持续下去，但我找不到任何可以解释它何时消失的东西。我特别想知道像这样的结构(来自 this old answer of mine ):

printf <- function(...) invisible(print(sprintf(...)))

凡外 invisible可能是不必要的(因为 print 已经将其返回值标记为不可见)。 withVisible()报告说这个函数的返回值是不可见的，但我不知道这是否由语言保证，或者只是它在当前实现中的工作方式。

Answer 1

通过反复试验:

# invisible
withVisible(invisible())$visible
[1] FALSE

### passing the invisible value through a function seems to
# preserve the invisibility
withVisible(identity(invisible()))$visible
[1] FALSE

# the <- operator just returns its arguments, so it confirms the above
withVisible(i <- invisible())$visible
[1] FALSE
# but the assigned value is no longer invisible
withVisible(i)$visible
[1] TRUE

### passing an invisible value as argument keeps the invisibility
f <- function(x) withVisible(x)$visible
f(1)
[1] TRUE
f(invisible(1))
[1] FALSE

### every other operation seems to cancel the invisibility.
# e.g. assigning an invisible value cancels the it
i <- invisible()
withVisible(i)$visible
[1] TRUE

withVisible(invisible(1) + 1)$visible
[1] TRUE