On 12/12/2013 2:08 PM, Karl Millar wrote:
According to http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Reserved-wordsif else repeat while function for in next break TRUE FALSE NULL Inf NaN NA NA_integer_ NA_real_ NA_complex_ NA_character_ ... ..1 ..2 etc. are all reserved keywords. However, in R 3.0.2 you can do things like: `if` <- function(cond, val1, val2) val2 after which if(TRUE) 1 else 2 returns 2. Similarly, users can change the implementation of `<-`, `(`, `{`, `||` and `&&`. Two questions: - Is this intended behaviour?
I would say yes.
- If so, would it be a good idea to change the language definition to prevent this?
I would say not. In the case of "if", what sophisticated users would expect to happen from
if (TRUE) 1 else 2 is that the `if` function will be called with arguments TRUE, 1, 2.
Doing so would both have the benefits that users could count on keywords having their normal interpretation, and allow R implementations to implement these more efficiently, including not having to lookup the symbol each time. It'd break any code that assumes that this is valid, but hopefully there's little or no code that does.
It would have those benefits, but it would be harder to prototype changes by actually replacing the `if` function. Implementations that want to optimize the calls have other ways to do it, e.g. the sorts of things the compiler does.
Duncan Murdoch ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
