无论参数是否带有括号，都可以正常工作

Question

我正在围绕某个包中的函数编写包装函数。该函数基本上为ggplot对象添加了新主题。但是在实现函数的方式中，主题参数需要指定为函数。但是我希望包装函数能够正常工作，而不管参数是指定为函数还是主题对象。我该怎么办？

以下是该功能，在什么条件下起作用，在什么条件下不起作用的玩具示例：

# loading needed libraries
library(ggplot2)

# creating basic plot on which themes are to be added
plot <- ggplot(mtcars, aes(wt, mpg)) + geom_point()

# first attempt at the function
theme_adder1 <- function(ggplot.obj, ggtheme = ggplot2::theme_bw) {
  # this can't be modified (because it was written by someone else)
  ggplot.obj + ggtheme() 
}

# this works
theme_adder1(ggplot.obj = plot, ggtheme = ggplot2::theme_grey)

# this doesn't work
theme_adder1(ggplot.obj = plot, ggtheme = ggplot2::theme_grey())
#> Error in ggtheme(): could not find function "ggtheme"

# checking classes
class(ggplot2::theme_bw())
#> [1] "theme" "gg"
class(ggplot2::theme_bw)
#> [1] "function"

因此，我想知道ggtheme对象-作为theme对象输入时是否可以通过某种方式转换为function对象，然后使用。但是我不确定该怎么做。

# second attempt at modifying function in a way that it will work irrespective
# of how the ggtheme argument is entered
theme_adder2 <- function(ggplot.obj, ggtheme = ggplot2::theme_bw) {
  if (class(ggtheme)[[1]] == "function") {
    ggplot.obj + ggtheme()
  } else if (class(ggtheme)[[1]] == "theme") {
    # what modifcation can be made here?
    ggtheme <- ??? 
    ggplot.obj + ggtheme()
  }
}

由reprex package（v0.2.0.9000）创建于2018-08-28。

Answer 1

尝试一下：

theme_adder2 <- function(ggplot.obj, ggtheme = ggplot2::theme_bw) {
  if (class(ggtheme)[[1]] == "function") {
    ggplot.obj + ggtheme()
  } else if (class(ggtheme)[[1]] == "theme") {
    # what modifcation can be made here?
      ggplot.obj + eval(ggtheme)
  }
}

还意识到您可以添加ggtheme而无需eval() ...

theme_adder2 <- function(ggplot.obj, ggtheme = ggplot2::theme_bw) {
  if (class(ggtheme)[[1]] == "function") {
    ggplot.obj + ggtheme()
  } else if (class(ggtheme)[[1]] == "theme") {
    # what modifcation can be made here?
      ggplot.obj + ggtheme
  }
}