Parser oddity with <- and =

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 at gmail.com
<mailto:murdoch.duncan at 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