R S4上下文中表达式的非标准评估

这是从他们函数中使用的函数中借用的

实际问题 调用S4方法（与标准R函数相反）时，如何延迟表达式的计算（例如通过arg

expr

指定）以“捕获”其内容

例子 请注意，

x_1

还不存在，这就是为什么我们要延迟对

expr

的评估，而只是“捕获”其内容

函数

captureExpression

：

captureExpression <- function(
  expr,
  caller_offset = 1,
  brackets = TRUE
) {
  out <- eval(substitute(substitute(expr)), parent.frame(caller_offset))
  if (brackets && class(out) != "{") {
    out <- substitute({CODE}, list(CODE = out))
  }
  out
}

captureExpression(x_1 * 2)
# {
#     x_1 * 2
# }

调用S4方法时，通过未计算的

expr

传递不起作用：

setGeneric(
  name = "bar",
  signature = c(
    "expr"
  ),
  def = function(
    expr,
    ...
  ) {
    standardGeneric("bar")       
  }
)
setMethod(
  f = "bar", 
  signature = signature(
    expr = "ANY"
  ), 
  definition = function(
    expr,
    ...
  ) {
  captureExpression(expr = expr, ...)    
})

bar(x_1 * 2)
# Error in bar(x_1 * 2) : 
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 2)
# Error in bar(x_1 * 2, caller_offset = 2) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 3)
# Error in bar(x_1 * 2, caller_offset = 3) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

应用S4方法：

setGeneric(
  name = "bar",
  signature = c(
    "expr"
  ),
  def = function(
    expr,
    ...
  ) {
    standardGeneric("bar")       
  }
)
setMethod(
  f = "bar", 
  signature = signature(
    expr = "ANY"
  ), 
  definition = function(
    expr,
    ...
  ) {
  captureExpression(expr = expr, ...)    
})

bar(x_1 * 2)
# Error in bar(x_1 * 2) : 
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 2)
# Error in bar(x_1 * 2, caller_offset = 2) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

bar(x_1 * 2, caller_offset = 3)
# Error in bar(x_1 * 2, caller_offset = 3) :
#   error in evaluating the argument 'expr' in selecting a method for function 'bar': Error: 
# object 'x_1' not found

我想这与方法分派的实际执行方式有关。然而，也许有一种方法可以做到这一点