Add support for using _ placeholder as a named argument on pipe RHS.

git-svn-id: https://svn.r-project.org/R/trunk@81858 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: luke

doc/NEWS.Rd

@@ -224,6 +224,12 @@
 
       \item \code{iconv} now allows \code{sub = "c99"} to use C99-style
       escapes for UTF-8 inputs which cannot be converted to encoding \code{to}.
+
+      \item In a forward pipe \code{|>} expression it is now possible to
+      use a named argument with the placeholder \code{_} in
+      the \code{rhs} call to specify where the \code{lhs} is to be
+      inserted.  The placeholder can only appear once on the \code{rhs}.
+
     }
   }
 

src/library/base/man/pipeOp.Rd

@@ -21,14 +21,17 @@
   A pipe expression passes, or pipes, the result of the left-hand side
   expression \code{lhs} to the right-hand side expression \code{rhs}.
 
-  %If the \code{rhs} expression is a call, then the
   The \code{lhs} is
   inserted as the first argument in the call. So \code{x |> f(y)} is
   interpreted as \code{f(x, y)}.
 
   To avoid ambiguities, functions in \code{rhs} calls may not be
   syntactically special, such as \code{+} or \code{if}.
 
+  It is also possible to use a named argument with the placeholder
+  \code{_} in the \code{rhs} call to specify where the \code{lhs} is to
+  be inserted.  The placeholder can only appear once on the \code{rhs}.
+  
   Pipe notation allows a nested sequence of calls to be written in a way
   that may make the sequence of processing steps easier to follow.
 
@@ -54,10 +57,13 @@ mtcars |> head()                      # same as head(mtcars)
 mtcars |> head(2)                     # same as head(mtcars, 2)
 mtcars |> subset(cyl == 4) |> nrow()  # same as nrow(subset(mtcars, cyl == 4))
 
-# passing the lhs into an argument other than the first, either:
+# to pass the lhs into an argument other than the first, either
+# use the _ placeholder with a named argument:
+mtcars |> subset(cyl == 4) |> lm(mpg ~ disp, data = _)
+# or use an anonymous function:
 mtcars |> subset(cyl == 4) |> (function(d) lm(mpg ~ disp, data = d))()
 mtcars |> subset(cyl == 4) |> (\(d) lm(mpg ~ disp, data = d))()
-# or explicitly naming the argument(s) before the "one":
+# or explicitly name the argument(s) before the "one":
 mtcars |> subset(cyl == 4) |> lm(formula = mpg ~ disp)
 
 # the pipe operator is implemented as a syntax transformation: