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

[Duncan Murdoch]

On 12/01/2021 3:52 p.m., Bill Dunlap wrote:
> '=>' can be defined as a function.  E.g., it could be the logical "implies"
> function:
>      > `=>` <- function(x, y) !x | y
>      > TRUE => FALSE
>      [1] FALSE
>      > FALSE => TRUE
>      [1] TRUE
> It might be nice then to have deparse() display it as an infix operator
> instead of the current prefix:
>      > deparse(quote(p => q))
>      [1] "`=>`(p, q)"
> There was a user who recently wrote asking for an infix operator like -> or
> => that would deparse nicely for use in some sort of model specification.

The precedence of it as an operator is determined by what makes sense in 
the pipe construction.  Currently precedence appears to be


:: :::	access variables in a namespace
$ @	component / slot extraction
[ [[	indexing
^	exponentiation (right to left)
- +	unary minus and plus
:	sequence operator
%any%	special operators (including %% and %/%)
* /	multiply, divide
+ -	(binary) add, subtract
< > <= >= == !=	ordering and comparison
!	negation
& &&	and
| ||	or
=>      PIPE BIND
|>      PIPE
~	as in formulae
-> ->>	rightwards assignment
<- <<-	assignment (right to left)
=	assignment (right to left)
?	help (unary and binary)

(Most of this is taken from ?Syntax, but I added the new operators in 
based on the gram.y file).  So

A & B => C & D

would appear to be parsed as

(A & B) => (C & D)

I think this also makes sense; do you?

Duncan Murdoch


> 
> When used with |>, the parser will turn the |> and => into an ordinary
> looking function call so deparsing is irrelevant.
>      > deparse(quote(x |> tmp => f(7,arg2=tmp)))
>      [1] "f(7, arg2 = x)"
> 
> -Bill
> 
> On Tue, Jan 12, 2021 at 12:01 PM Dirk Eddelbuettel <edd using debian.org> wrote:
> 
>>
>> On 12 January 2021 at 20:38, Iñaki Ucar wrote:
>> | On Tue, 12 Jan 2021 at 20:23, <luke-tierney using uiowa.edu> wrote:
>> | >
>> | > After some discussions we've settled on a syntax of the form
>> | >
>> | >      mtcars |> subset(cyl == 4) |> d => lm(mpg ~ disp, data = d)
>> | >
>> | > to handle cases where the pipe lhs needs to be passed to an argument
>> | > other than the first of the function called on the rhs. This seems a
>> | > to be a reasonable balance between making these non-standard cases
>> | > easy to see but still easy to write. This is now committed to R-devel.
>> |
>> | Interesting. Is the use of "d =>" restricted to pipelines? In other
>> | words, I think that it shouldn't be equivalent to "function(d)", i.e.,
>> | that this:
>> |
>> | x <- d => lm(mpg ~ disp, data = d)
>> |
>> | shouldn't work.
>>
>> Looks like your wish was already granted:
>>
>>    > mtcars |> subset(cyl == 4) |> d => lm(mpg ~ disp, data = d)
>>
>>    Call:
>>    lm(formula = mpg ~ disp, data = subset(mtcars, cyl == 4))
>>
>>    Coefficients:
>>    (Intercept)         disp
>>         40.872       -0.135
>>
>>    > d => lm(mpg ~ disp, data = d)
>>    Error in `=>`(d, lm(mpg ~ disp, data = d)) : could not find function "=>"
>>    > x <- d => lm(mpg ~ disp, data = d)
>>    Error in `=>`(d, lm(mpg ~ disp, data = d)) : could not find function "=>"
>>    >
>>
>> Dirk
>>
>> --
>> https://dirk.eddelbuettel.com | @eddelbuettel | edd using debian.org
>>
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