[Rd] [External] Re: New pipe operator

Duncan Murdoch murdoch@dunc@n @end|ng |rom gm@||@com
Sun Dec 6 16:32:56 CET 2020 

On 06/12/2020 9:43 a.m., Dénes Tóth wrote:
> Dear Luke,
> 
> In the meantime I checked the R-syntax branch and the docs; they are
> very helpful. I would also like to thank you for putting effort into
> this feature. Keeping it at the syntax level is also a very smart
> decision. However, the current API might not exploit the full power of
> the basic idea.
> 
> 1) Requiring either an anonymous function or a function call, but not
> allowing for symbols which point to functions is inconsistent and will
> be misleading for non-experts.
> 
> foo <- function(x) x
> identical(foo, function(x) x)
> 
> mtcars |> foo               #bang!
> mtcars |> function(x) x     #fine?

You are missing the point.  The value of the RHS is irrelevant to the 
transformation.  All that matters is its form.  So "foo" and 
"function(x) x" are completely different things, even if identical() 
thinks their value is the same.

It's also true that "foo()" and "function(x) x" are completely 
different, but they are well-defined forms:  one is a call, the other is 
an anonymous function definition.

Accepting a plain "foo" would add a third form (a name), which might 
make sense, but hardly gains anything: whereas dropping the anonymous 
function definition costs quite a bit.  Without special-casing anonymous 
function definitions you'd need to enter

mtcars |> (function(x) x)()

or

mtcars |> (\(x) x)()

which are both quite difficult to read.

Duncan Murdoch

> 
> You stated in :
> "
> Another variation supported by the implementation is that a symbol on
> the RHS is interpreted as the name of a function to call with the LHS
> as argument:
> 
> ```r
>   > quote(x |> f)
> f(x)
> ```
> "
> 
> So clearly this is not an implementation issue but a design decision.
> 
> As a remedy, two different pipe operators could be introduced:
> 
> LHS |> RHS    -> RHS is treated as a function call
> LHS |>> RHS   -> RHS is treated as a function
> 
> If |>> is used, it would not matter which notation is used for the RHS
> expression; the parser would assume it evaluates to a function.
> 
> 2) Simplified lambda expression:
> IMHO in the vast majority of use cases, this is used for single-argument
> functions, so parenthesis would not be required. Hence, both forms would
> be valid and equivalent:
> 
> \x x + 1
> \(x) x + 1
> 
> 
> 3) Function composition:
> Allowing for concise composition of functions would be a great feature.
> E.g., instead of
> 
> foo <- function(x) print(mean(sqrt(x), na.rm = TRUE), digits = 2)
> 
> or
> 
> foo <- \x {x |> sqrt() |> mean(na.rm = TRUE) |> print(digits = 2)}
> 
> one could write
> 
> foo <- \x |> sqrt() |> mean(na.rm = TRUE) |> print(digits = 2)
> 
> So basically if the lambda argument is followed by a pipe operator, the
> pipe chain is transformed to a function body where the first lambda
> argument is inserted into the first position of the pipeline.
> 
> 
> Best,
> Denes
> 
> 
> On 12/5/20 7:10 PM, luke-tierney using uiowa.edu wrote:
>> We went back and forth on this several times. The key advantage of
>> requiring parentheses is to keep things simple and consistent.  Let's
>> get some experience with that. If experience shows requiring
>> parentheses creates too many issues then we can add the option of
>> dropping them later (with special handling of :: and :::). It's easier
>> to add flexibility and complexity than to restrict it after the fact.
>>
>> Best,
>>
>> luke
>>
>> On Sat, 5 Dec 2020, Hugh Parsonage wrote:
>>
>>> I'm surprised by the aversion to
>>>
>>> mtcars |> nrow
>>>
>>> over
>>>
>>> mtcars |> nrow()
>>>
>>> and I think the decision to disallow the former should be
>>> reconsidered.  The pipe operator is only going to be used when the rhs
>>> is a function, so there is no ambiguity with omitting the parentheses.
>>> If it's disallowed, it becomes inconsistent with other treatments like
>>> sapply(mtcars, typeof) where sapply(mtcars, typeof()) would just be
>>> noise.  I'm not sure why this decision was taken
>>>
>>> If the only issue is with the double (and triple) colon operator, then
>>> ideally `mtcars |> base::head` should resolve to `base::head(mtcars)`
>>> -- in other words, demote the precedence of |>
>>>
>>> Obviously (looking at the R-Syntax branch) this decision was
>>> considered, put into place, then dropped, but I can't see why
>>> precisely.
>>>
>>> Best,
>>>
>>>
>>> Hugh.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Sat, 5 Dec 2020 at 04:07, Deepayan Sarkar
>>> <deepayan.sarkar using gmail.com> wrote:
>>>>
>>>> On Fri, Dec 4, 2020 at 7:35 PM Duncan Murdoch
>>>> <murdoch.duncan using gmail.com> wrote:
>>>>>
>>>>> 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 is really not that far off from
>>>>
>>>> mtcars |> subset(cyl == 4) |> \(.) lm(mpg ~ disp, data = .)
>>>>
>>>> once you get used to it.
>>>>
>>>> One consequence of the implementation is that it's not clear how
>>>> multiple occurrences of the placeholder would be interpreted. With
>>>> magrittr,
>>>>
>>>> sort(runif(10)) %>% ecdf(.)(.)
>>>> ## [1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
>>>>
>>>> This is probably what you would expect, if you expect it to work at
>>>> all, and not
>>>>
>>>> ecdf(sort(runif(10)))(sort(runif(10)))
>>>>
>>>> There would be no such ambiguity with anonymous functions
>>>>
>>>> sort(runif(10)) |> \(.) ecdf(.)(.)
>>>>
>>>> -Deepayan
>>>>
>>>>> 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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>>>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>>>>
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