[Rd] New pipe operator

Gabriel Becker g@bembecker @end|ng |rom gm@||@com
Mon Dec 7 03:23:47 CET 2020 

Hi Gabor,

On Sun, Dec 6, 2020 at 3:22 PM Gabor Grothendieck <ggrothendieck using gmail.com>
wrote:

> I understand very well that it is implemented at the syntax level;
> however, in any case the implementation is irrelevant to the principles.
>
> Here a similar example to the one I gave before but this time written out:
>
> This works:
>
>   3 |> function(x) x + 1
>
> but this does not:
>
>   foo <- function(x) x + 1
>   3 |> foo
>
> so it breaks the principle of functions being first class objects.  foo
> and its
> definition are not interchangeable.


I understood what you meant as well.

The issue is that neither foo nor its definition are being operated on, or
even exist within the scope of what |> is defined to do. You are used to
magrittr's %>% where arguably what you are saying would be true. But its
not here, in my view.

Again, I think the issue is that |>, in as much as it "operates" on
anything at all (it not being a function, regardless of appearances),
operates on call expression objects, NOT on functions, ever.

function(x) x *parses to a call expression *as does RHSfun(), while RHSfun does
not, it parses to a name, *regardless of whether that symbol will
eventually evaluate to a closure or not.*

So in fact, it seems to me that, technically, all name symbols are being
treated exactly the same (none are allowed, including those which will
lookup to functions during evaluation), while all* call expressions are
also being treated the same. And again, there are no functions anywhere in
either case.

* except those that include that the parser flags as syntactically special.


> You have
> to write 3 |> foo() but don't have to write 3 |> (function(x) x + 1)().
>

I think you should probably be careful what you wish for here. I'm not
involved with this work and do not speak for any of those who were, but the
principled way to make that consistent while remaining entirely in the
parser seems very likely to be to require the latter, rather than not
require the former.


> This isn't just a matter of notation, i.e. foo vs foo(), but is a
> matter of breaking
> the way R works as a functional language with first class functions.
>

I don't agree. Consider `+`

Having

foo <- get("+") ## note no `` here
foo(x,y)

parse and work correctly while

+(x,y)

 does not does not mean + isn't a function or that it is a "second class
citizen", it simply means that the parser has constraints on the syntax for
writing code that calls it that calling other functions are not subject to.
The fact that such *syntactic* constraints can exist proves that there is
not some overarching inviolable principle being violated here, I think. Now
you may say "well thats just the parser, it has to parse + specially
because its an operator with specific precedence etc". Well, the same exact
thing is true of |> I think.

Best,
~G

>
> On Sun, Dec 6, 2020 at 4:06 PM Gabriel Becker <gabembecker using gmail.com>
> wrote:
> >
> > Hi Gabor,
> >
> > On Sun, Dec 6, 2020 at 12:52 PM Gabor Grothendieck <
> ggrothendieck using gmail.com> wrote:
> >>
> >> I think the real issue here is that functions are supposed to be
> >> first class objects in R
> >> or are supposed to be and |> would break that if if is possible
> >> to write function(x) x + 1 on the RHS but not foo (assuming foo
> >> was defined as that function).
> >>
> >> I don't think getting experience with using it can change that
> >> inconsistency which seems serious to me and needs to
> >> be addressed even if it complicates the implementation
> >> since it drives to the heart of what R is.
> >>
> >
> > With respect I think this is a misunderstanding of what is happening
> here.
> >
> > Functions are first class citizens. |> is, for all intents and purposes,
> a macro.
> >
> > LHS |> RHS(arg2=5)
> >
> > parses to
> >
> > RHS(LHS, arg2 = 5)
> >
> > There are no functions at the point in time when the pipe transformation
> happens, because no code has been evaluated. To know if a symbol is going
> to evaluate to a function requires evaluation which is a step entirely
> after the one where the |> pipe is implemented.
> >
> > Another way to think about it is that
> >
> > LHS |> RHS(arg2 = 5)
> >
> > is another way of writing RHS(LHS, arg2 = 5), NOT R code that is (or
> even can be) evaluated.
> >
> >
> > Now this is a subtle point that only really has implications in as much
> as it is not the case for magrittr pipes, but its relevant for discussions
> like this, I think.
> >
> > ~G
> >
> >> On Sat, Dec 5, 2020 at 1:08 PM Gabor Grothendieck
> >> <ggrothendieck using gmail.com> wrote:
> >> >
> >> > The construct utils::head  is not that common but bare functions are
> >> > very common and to make it harder to use the common case so that
> >> > the uncommon case is slightly easier is not desirable.
> >> >
> >> > Also it is trivial to write this which does work:
> >> >
> >> > mtcars %>% (utils::head)
> >> >
> >> > On Sat, Dec 5, 2020 at 11:59 AM Hugh Parsonage <
> hugh.parsonage using gmail.com> 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
> >> > > > > >>
> >> > > > > >> ______________________________________________
> >> > > > > >> R-devel using r-project.org mailing list
> >> > > > > >> https://stat.ethz.ch/mailman/listinfo/r-devel
> >> > > > > >
> >> > > > > > ______________________________________________
> >> > > > > > R-devel using r-project.org mailing list
> >> > > > > > https://stat.ethz.ch/mailman/listinfo/r-devel
> >> > > > > >
> >> > > > >
> >> > > > > ______________________________________________
> >> > > > > R-devel using r-project.org mailing list
> >> > > > > https://stat.ethz.ch/mailman/listinfo/r-devel
> >> > > >
> >> > > > ______________________________________________
> >> > > > R-devel using r-project.org mailing list
> >> > > > https://stat.ethz.ch/mailman/listinfo/r-devel
> >> > >
> >> > > ______________________________________________
> >> > > R-devel using r-project.org mailing list
> >> > > https://stat.ethz.ch/mailman/listinfo/r-devel
> >> >
> >> >
> >> >
> >> > --
> >> > Statistics & Software Consulting
> >> > GKX Group, GKX Associates Inc.
> >> > tel: 1-877-GKX-GROUP
> >> > email: ggrothendieck at gmail.com
> >>
> >>
> >>
> >> --
> >> Statistics & Software Consulting
> >> GKX Group, GKX Associates Inc.
> >> tel: 1-877-GKX-GROUP
> >> email: ggrothendieck at gmail.com
> >>
> >> ______________________________________________
> >> R-devel using r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
>

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