[Rd] Parser oddity with <- and =

[Duncan Murdoch]

Sorry, I wrote in a sloppy way.  The parsing I see is just what you saw. 
  It's the evaluation of that expression that tries to call `<-<-`:

 > a <- 1
 > a <- b = c
Error in a <- b = c : could not find function "<-<-"

This happens because R is trying to make an assignment using = with a 
LHS that is the function call a <- b.

Duncan Murdoch


On 04/02/2022 3:28 p.m., Bill Dunlap wrote:
> In R-4.1.2 and R-devel from two weeks ago I do not get the `<-<-`:
> 
>      > str.language(parse(text = "a <- b = c"))
>     expression: structure(expression(a <- b = c), sr ...
>        language: a <- b = c
>          symbol: =
>          language: a <- b
>            symbol: <-
>            symbol: a
>            symbol: b
>          symbol: c
>      > identical(parse(text = "a <- b = c")[[1]], parse(text = "`=`(
>     `<-`(a, b), c)")[[1]])
>     [1] TRUE
> 
> str.language() is a rudimentary parse tree displayer:
> 
>     str.language <- function(expr, name = "", indent = 0)
>     {
>          trim... <- function(string, width.cutoff) {
>              if (nchar(string) > width.cutoff) {
>                  string <- sprintf("%.*s ...", width.cutoff-4, string)
>              }
>              string
>          }
>          cat(sep="", rep("  ", indent), typeof(expr), ": ",
>              if(length(name)==1 && nzchar(name)) { paste0(name, " = ") },
>              trim...(deparse1(expr, width.cutoff=40), width.cutoff=40),
>              "\n")
>          if (is.function(expr)) {
>              str.language(formals(expr), name="[formals]", indent =
>     indent + 1)
>              str.language(body(expr), name="[body]", indent = indent + 1)
>          } else if (is.recursive(expr)) {
>              expr <- as.list(expr)
>              nms <- names(expr)
>              for (i in seq_along(expr)) {
>                  str.language(expr[[i]], name=nms[[i]], indent = indent + 1)
>              }
>          }
>          invisible(expr)
>     }
> 
> 
> -Bill
> 
> On Fri, Feb 4, 2022 at 9:34 AM Duncan Murdoch <murdoch.duncan using gmail.com 
> <mailto:murdoch.duncan using gmail.com>> wrote:
> 
>     Here's an odd parse:
> 
>          a <- b = 1
> 
>     This appears to be parsed as
> 
>          `<-<-`(a, b, 1)
> 
>     instead of being equivalent to
> 
>          a <- b <- 1
> 
>     I wonder if that's intentional?
> 
>     (This showed up at https://stackoverflow.com/q/70989067/2554330
>     <https://stackoverflow.com/q/70989067/2554330>, where
>     it caused a lot of confusion.  I think the original intent was that `a`
>     would be a macro holding `b = 1`, but I'm not sure of that.)
> 
>     Duncan Murdoch
> 
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