[Rd] [External] Re: New pipe operator

Duncan Murdoch murdoch@dunc@n @end|ng |rom gm@||@com
Sat Dec 5 02:03:23 CET 2020

On 04/12/2020 2:26 p.m., luke-tierney using uiowa.edu wrote:
> On Fri, 4 Dec 2020, Dénes Tóth wrote:
>> On 12/4/20 3:05 PM, Duncan Murdoch wrote:
>>> ...
>>> 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 support the idea of using an underscore (_) as the placeholder symbol.
> I strongly oppose adding a placeholder. Allowing for an optional
> placeholder significantly complicates both implementing and explaining
> the semantics. For a simple syntax transformation to be viable it
> would also require some restrictions, such as only allowing a
> placeholder as a top level argument and only once. Checking that these
> restrictions are met, and accurately signaling when they are not with
> reasonable error messages, is essentially an unsolvable problem given
> R's semantics.

I don't think you read my suggestion, but that's okay:  you're 
maintaining it, not me.

Duncan Murdoch

> The case where the LHS is to be passed as something other than the
> first argument is unusual. For me, having that case stand out by using
> a function expression makes it much easier to see and so makes the
> code easier to understand. As a wearer of progressive bifocals
> and someone whose screen is not always free of small dust particles,
> having to spot the non-standard pipe stages by seeing a placeholder,
> especially a . placeholder, is be a bug, not a feature.
> Best,
> luke
>> Syntactic sugars work the the best if 1) they require less keystrokes and/or
>> 2) are easier to read compared to the "normal" syntax, and 3) can not lead to
>> unexpected bugs (which is a major problem with the magrittr pipe). Using '_'
>> fulfills all of these criteria since '_' can not clash with any variable in
>> the environment.
>> Denes
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