[Rd] Pipe operator status, placeholders?

[Benjamin Redelings]

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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