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Parser oddity with <- and =

3 messages · Duncan Murdoch, Bill Dunlap

Original
Duncan Murdoch
# 
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, 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
Bill Dunlap
# 
In R-4.1.2 and R-devel from two weeks ago I do not get the `<-<-`:
expression: structure(expression(a <- b = c), sr ...
  language: a <- b = c
    symbol: =
    language: a <- b
      symbol: <-
      symbol: a
      symbol: b
    symbol: c
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>
wrote:

  
  


  


      

    

    

      
      

        


  

        

      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: