[Rd] New pipe operator

[Duncan Murdoch]

On 04/12/2020 8:13 a.m., Hiroaki Yutani wrote:
>>   Error: function '::' not supported in RHS call of a pipe
> 
> To me, this error looks much more friendly than magrittr's error.
> Some of them got too used to specify functions without (). This
> is OK until they use `::`, but when they need to use it, it takes
> hours to figure out why
> 
> mtcars %>% base::head
> #> Error in .::base : unused argument (head)
> 
> won't work but
> 
> mtcars %>% head
> 
> works. I think this is a too harsh lesson for ordinary R users to
> learn `::` is a function. I've been wanting for magrittr to drop the
> support for a function name without () to avoid this confusion,
> so I would very much welcome the new pipe operator's behavior.
> Thank you all the developers who implemented this!

I agree, it's an improvement on the corresponding magrittr error.

I think the semantics of not evaluating the RHS, but treating the pipe 
as purely syntactical is a good decision.

I'm not sure I like the recommended way to pipe into a particular argument:

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

or

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

both of which are equivalent to

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

It's tempting to suggest it should allow something like

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

which would be expanded to something equivalent to the other versions: 
but that makes it quite a bit more complicated.  (Maybe _ or \. should 
be used instead of ., since those are not legal variable names.)

I don't think there should be an attempt to copy magrittr's special 
casing of how . is used in determining whether to also include the 
previous value as first argument.

Duncan Murdoch


> 
> Best,
> Hiroaki Yutani
> 
> 2020年12月4日(金) 20:51 Duncan Murdoch <murdoch.duncan using gmail.com>:
>>
>> Just saw this on the R-devel news:
>>
>>
>> R now provides a simple native pipe syntax ‘|>’ as well as a shorthand
>> notation for creating functions, e.g. ‘\(x) x + 1’ is parsed as
>> ‘function(x) x + 1’. The pipe implementation as a syntax transformation
>> was motivated by suggestions from Jim Hester and Lionel Henry. These
>> features are experimental and may change prior to release.
>>
>>
>> This is a good addition; by using "|>" instead of "%>%" there should be
>> a chance to get operator precedence right.  That said, the ?Syntax help
>> topic hasn't been updated, so I'm not sure where it fits in.
>>
>> There are some choices that take a little getting used to:
>>
>>   > mtcars |> head
>> Error: The pipe operator requires a function call or an anonymous
>> function expression as RHS
>>
>> (I need to say mtcars |> head() instead.)  This sometimes leads to error
>> messages that are somewhat confusing:
>>
>>   > mtcars |> magrittr::debug_pipe |> head
>> Error: function '::' not supported in RHS call of a pipe
>>
>> but
>>
>> mtcars |> magrittr::debug_pipe() |> head()
>>
>> works.
>>
>> Overall, I think this is a great addition, though it's going to be
>> disruptive for a while.
>>
>> Duncan Murdoch
>>
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