[External] brief update on the pipe operator in R-devel

It turns out that allowing a bare function expression on the
right-hand side (RHS) of a pipe creates opportunities for confusion
and mistakes that are too risky. So we will be dropping support for
this from the pipe operator.

The case of a RHS call that wants to receive the LHS result in an
argument other than the first can be handled with just implicit first
argument passing along the lines of

   mtcars |> subset(cyl == 4) |> (\(d) lm(mpg ~ disp, data = d))()

It was hoped that allowing a bare function expression would make this
more convenient, but it has issues as outlined below. We are exploring
some alternatives, and will hopefully settle on one soon after the
holidays.

The basic problem, pointed out in a comment on Twitter, is that in
expressions of the form

   1 |> \(x) x + 1 -> y
   1 |> \(x) x + 1 |> \(y) x + y

everything after the \(x) is parsed as part of the body of the
function.  So these are parsed along the lines of

   1 |> \(x) { x + 1 -> y }
   1 |> \(x) { x + 1 |> \(y) x + y }

In the first case the result is assigned to a (useless) local
variable.  Someone writing this is more likely to have intended to
assign the result to a global variable, as this would:

   (1 |> \(x) x + 1) -> y

In the second case the 'x' in 'x + y' refers to the local variable 'x'
in the first RHS function. Someone writing this is more likely to have
meant

   (1 |> \(x) x + 1) |> \(y) x + y

with 'x' in 'x + y' now referring to a global variable:

   > x <- 2
   > 1 |> \(x) x + 1 |> \(y) x + y

   [1] 3

   > (1 |> \(x) x + 1) |> \(y) x + y

   [1] 4

These issues arise with any approach in R that allows a bare function
expression on the RHS of a pipe operation. It also arises in other
languages with pipe operators. For example, here is the last example
in Julia:

   julia> x = 2
   2
   julia> 1 |> x -> x + 1 |> y -> x + y
   3
   julia> ( 1 |> x -> x + 1 ) |> y -> x + y
   4

Even though proper use of parentheses can work around these issues,
the likelihood of making mistakes that are hard to track down is too
high. So we will disallow the use of bare function expressions on the
right hand side of a pipe.

Best,

luke