英文:
How can I redefine '+' inside of a closure?
问题
首先，我知道这是愚蠢的，实际情况下永远不应该这样做。我也知道，即使在实践中这样做，你应该使用面向对象编程。我只是在这里为了证明一个观点。
我想重新定义 +。我想让下面这段代码工作：
'+' <- function(x, y) x + y + 2
允许像下面这样的代码：
2 + 2
返回 6。
目前，它不起作用，因为它会导致无限递归。为了解决这个问题，我想使用闭包，但我不知道该怎么做。显然，我想要类似这样的东西：
SillyPlusGenerator <- function() {'+' <- function(x, y) x + y + 2}
但这不会让我在重新定义 + 的环境中计算 2 + 2，也不会告诉 SillyPlusGenerator 在父环境中查找 +。我知道我可以尝试使用 eval 并直接使用环境，但我确信闭包就是我所需要的。我错过了哪一步？
英文:
First of all, I know that this is stupid and never should be done in practice. I also know that even when doing it in practice, you ought to do it with OOP. I'm just doing it to prove a point.
I want to redefine '+'. I want
'+' <- function(x, y) x + y + 2
to work. Allowing for code like
2 + 2
to return 6.
Currently, it doesn't work because it causes an infinite recursion. To solve this, I want to use a closure, but I'm at a loss as to how. I obviously want something like:
SillyPlusGenerator <- function() {'+' <- function(x, y) x + y + 2}
but that doesn't give me a way to evaluate 2 + 2 in an environment where + is redefined and it doesn't tell SillyPlusGenerator to look up + in the parent environment. I know that I could play around with eval and use environments directly, but I feel sure that closures are all that I need.
What step have I missed?
答案1
得分: 1
以下是翻译好的部分：
"The answer shows a couple of quick ways To solve this, but does not attempt to use closures.
To avoid the infinite recursion, you can:
- Call the normal base R
+function explicitly within the custom function (noting that+can be called as you would with a standard function e.g.+(2,2)).
`+` <- function(x, y) (base::`+`)((base::`+`)(x, y), 2)
# and a variation of:
`+` <- function(x, y) Reduce(base::`+`, c(x, y, 2))
- Redefine the
+function within the custom function
`+` <- function(x, y) { `+` = base::`+` ; x + y + 2}
"
英文:
The answer shows a couple of quick ways "To solve this", but does not attempt to use closures.
To avoid the infinite recursion, you can:
- Call the normal base R <code>`+`</code> function explicitly within the custom function (noting that <code>`+`</code> can be called as you would with a standard function e.g. <code>`+`(2,2)</code>).
`+` <- function(x, y) (base::`+`)((base::`+`)(x, y), 2)
# and a variation of:
`+` <- function(x, y) Reduce(base::`+`, c(x, y, 2))
- Redefine the <code>`+`</code> function within the custom function
`+` <- function(x, y) { `+` = base::`+` ; x + y + 2}
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