[Rd] [ on call

[Duncan Murdoch]

On 07/07/2015 6:33 PM, Mick Jordan wrote:
> I'm rather puzzled by this behavior:
> 
> e
> export("caption<-", "caption", "label", "label<-", "align<-",
>      "align", "digits<-", "digits", "display<-", "display", "xtable",
>      "print.xtable", "toLatex.xtable")
>  > e[[1L]]
> e[[1L]]
> export
>  > e[-1L]
> e[-1L]
> "caption<-"("caption", "label", "label<-", "align<-", "align",
>      "digits<-", "digits", "display<-", "display", "xtable", 
> "print.xtable",
>      "toLatex.xtable")
> 
> I'm not at all clear what should result from removing the first element, 
> i.e. the 'export', but I would not expect the first argument to be 
> promoted into the function position. I guess I would expect a coercion 
> to list or pairlist first, after which the [-1L] would produce a 
> meaningful result on that coercion. In any event I do observe that 
> as.character(e[-1L]) produces the expected result:
> 
> as.character(e[-1L])
>   [1] "caption<-"      "caption"        "label" "label<-"
>   [5] "align<-"        "align"          "digits<-" "digits"
>   [9] "display<-"      "display"        "xtable" "print.xtable"
> [13] "toLatex.xtable"
> 
> This code is from parseNamespaceFile (on the xtable package).

In R, language objects are specially marked pairlists.  The first
element is the function to call, the second is its first argument, etc.
So if you delete the first element and do nothing else, that promotes
the first argument to the function to call.

Duncan Murdoch