[Rd] Status of reserved keywords and builtins

[Duncan Murdoch]

On 13-12-12 3:22 PM, Duncan Murdoch wrote:
> 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-words
>>
>>     if 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.

One other comment:

A package could replace `if` for the user, but it would not affect any 
packages, unless those packages chose to import the new definition.  So 
this is a risk to people who write lots of code in scripts and 
indiscriminately attach packages, but not much risk to people who put 
their code in packages and are careful about what they import.  And 
those people with the scripts are already at risk due to changes to 
functions like mean(), sum(), c(), etc.  So they shouldn't do that. 
That's been the standard advice for a long time...

Duncan Murdoch