[Rd] Pipe operator status, placeholders?

[Duncan Murdoch]

On 19/04/2022 6:55 p.m., Simon Urbanek wrote:
> Ben,
> 
> I think you considered only part of Peter's response. Placeholders can safely only work for the first call, hence at the top level. Anything below may not do what you think as you'd have to skip frames and suddenly things can have entirely different meaning since you're not evaluating in the scope of the preceding call. That is also the reason why only named arguments are allowed, because if it was not the case then you might be tempted to write x |> _$foo[1] which looks legit at a first glance, but is no longer at the top level (since it is `[`(`$`(_, foo), 1)) and thus not valid.

The R pipe is purely syntactic sugar, it just transforms expressions.  I 
think the real reason not to allow _ to be deeply nested in an 
expression is that it would make parsing really hard.  If you have

  x |> { some really huge expression }

then the parser would have to parse the huge expression and search it 
for underscores to see what to do with x.  With the current rule, the 
search is much easier, it's just at the top level.

There are probably cases where deeply nested underscores would be 
ambiguous, e.g. if that huge expression contained a pipe operator 
itself, who gets the substitution?

The other limitation of the transformation approach is that _ can only 
occur once.  magrittr evaluates x and puts the value in where it sees a 
dot, so this works to print 2 once and give a value of 4:

   print(2) %>% `+`(., .)

It's equivalent to

   *tmp* <- print(2)
   *tmp* + *tmp*

However, you'd have the print executed twice in

   print(2) |> `+`(_, _)

(if such was allowed), because it would be equivalent to

   print(2) + print(2)

Duncan Murdoch

> 
> Cheers,
> Simon
> 
> 
> 
>> On Apr 20, 2022, at 12:43 AM, Benjamin Redelings <benjamin.redelings using gmail.com> wrote:
>>
>> Thanks to you and Lionel for the info!  I wasn't sure if there was a private core developers list, or if I was just looking in the wrong place.
>>
>> 1. Its good to know that the only reason not to allow _ in positional arguments is that its easy to miss.  Personally, I would like to be able to write f(x, _), but its not a big deal.
>>
>> Is the idea that when you see
>>
>>      x |> f(x, y, _, z, w)
>>
>> its hard for the eye to scan the RHS and find the _?
>>
>> Hmm.... I notice that a lot of languages (i.e. Haskell) use _ as a wildcard pattern, and I don't recall any complaints about it being hard to see.
>>
>> 2. I can see how there would be issues with placeholders that aren't at the top level... although I'm not sure precisely what they are.  Any hints? :-)  I did briefly look at the parser/grammar file...
>>
>> Thanks again for the info!
>>
>> -BenRI
>>
>> On 4/19/22 3:24 AM, peter dalgaard wrote:
>>> You probably want Luke Tierney for the full story, but what I gather from the deliberations (on the private R-core list), there are issues with how non-funcall syntax like lm(....) |> _$coef[2] should work. This, in turn, has to do with wanting to have the placeholder occur only as a toplevel substitution (i.e. "["("$"(_, coef), 2) is a no-go. And the reason for that has to do with the way the pipe works in the absense of placeholder, e.g. the parser gets confused by
>>>
>>>> x |> f(g(x=_))
>>> Error in f(x, g(x = "_")) : invalid use of pipe placeholder
>>>
>>> -pd
>>>
>>>> On 17 Apr 2022, at 01:04 , Benjamin Redelings <benjamin.redelings using gmail.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> I see that R 4.2 adds the underscore _ as a placeholder for the new forward pipe operator |> , but only for named arguments. The reason why placeholders for position arguments was NOT added isn't clear to me, so I've been looking for the discussion around the introduction of the placeholder.
>>>>
>>>> By searching subject lines in the r-devel mailing list archive, I've found
>>>>
>>>>      https://stat.ethz.ch/pipermail/r-devel/2021-April/080646.html
>>>>
>>>> https://stat.ethz.ch/pipermail/r-devel/2021-January/080396.html
>>>>
>>>> https://stat.ethz.ch/pipermail/r-devel/2020-December/080173.html and following messages
>>>>
>>>> but not much else.
>>>>
>>>> 1. Am I looking in the wrong place?
>>>>
>>>> 2. What is the reasoning behind allowing _ as a placeholder only for named arguments?
>>>>
>>>> take care,
>>>>
>>>> -BenRI
>>>>
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