brief update on the pipe operator in R-devel

f <- function(x, y) x + 10*y
3 |> x => f(x, x)

3 |> x => f(1+x, 1)

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

--
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu