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

[Bill Dunlap]

If
    3 |> x => f(x, y=x)
were allowed then I think that
     runif(1) |> x => f(x, y=x)
be parsed as
     f(runif(1), y=runif(1))
so runif(1) would be evaluated twice, leading to incorrect results from f().

-Bill

On Fri, Jan 15, 2021 at 2:16 PM Avi Gross via R-devel <r-devel using r-project.org>
wrote:

> Gabor,
>
> Although it might be nice if all imagined cases worked, there are many
> ways to work around and get the results you want.
>
> You may want to consider that it is easier to recognize the symbol you use
> (x in the examples) if it is alone and used only exactly once and it the
> list of function arguments.  If you want the x used multiple times, you can
> make a function that accepts the x once and then invokes another function
> and reuses the x as often as needed. Similarly for 1+x.
>
> I do not know if the above choice was made to make it easier and faster to
> apply the above, or to avoid possible bad edge cases. Have you tested other
> ideas like:
>
>         3 |> x => f(x=5)
> Or
>         3 |> x => f(x, y=x)
>
> I mean ones where a default is supplied, not that it makes much sense here?
>
> I am thinking of the concept of substitution as is often done for text or
> symbols. Often the substitution is done for the first instance found unless
> you specify you want a global change. In your examples, if only the first
> use of x would be replaced, the second naked x being left alone would be an
> error. If all instances were changed, what anomalies might happen? Giving a
> vector of length 1 containing the number 3 seems harmless enough to
> duplicate. But the pipeline can send all kinds of interesting data
> structures through including data.frames and arbitrary objects.
>
>
> -----Original Message-----
> From: R-devel <r-devel-bounces using r-project.org> On Behalf Of Gabor
> Grothendieck
> Sent: Friday, January 15, 2021 7:28 AM
> To: Tierney, Luke <luke-tierney using uiowa.edu>
> Cc: R-devel using r-project.org
> Subject: Re: [Rd] brief update on the pipe operator in R-devel
>
> These are documented but still seem like serious deficiencies:
>
> > f <- function(x, y) x + 10*y
> > 3 |> x => f(x, x)
> Error in f(x, x) : pipe placeholder may only appear once
>
> > 3 |> x => f(1+x, 1)
> Error in f(1 + x, 1) :
>   pipe placeholder must only appear as a top-level argument in the RHS call
>
> Also note:
>
>  ?"=>"
> No documentation for ‘=>’ in specified packages and libraries:
> you could try ‘??=>’
>
> On Tue, Dec 22, 2020 at 5:28 PM <luke-tierney using uiowa.edu> wrote:
> >
> > 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 using uiowa.edu
> > Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
> >
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>
>
>
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