[Rd] vctrs: a type system for the tidyverse

[Gabe Becker]

Hadley,

Looks interesting and like a fun project from what you said in the email (I
don't have time right now to dig deep into the readme) A few thoughts.

First off, you are using the word "type" throughout this email; You seem to
mean class (judging by your Date and factor examples, and the fact you
mention S3 dispatch) as opposed to type in the sense of what is returned by
R's  typeof() function. I think it would be clearer if you called it class
throughout unless that isn't actually what you mean (in which case I would
have other questions...)

More thoughts inline.

On Mon, Aug 6, 2018 at 9:21 AM, Hadley Wickham <h.wickham using gmail.com> wrote:

> Hi all,
>
> I wanted to share with you an experimental package that I’m currently
> working on: vctrs, <https://github.com/r-lib/vctrs>. The motivation for
> vctrs is to think deeply about the output “type” of functions like
> `c()`, `ifelse()`, and `rbind()`, with an eye to implementing one
> strategy throughout the tidyverse (i.e. all the functions listed at
> <https://github.com/r-lib/vctrs#tidyverse-functions>). Because this is
> going to be a big change, I thought it would be very useful to get
> comments from a wide audience, so I’m reaching out to R-devel to get
> your thoughts.
>
> There is quite a lot already in the readme
> (<https://github.com/r-lib/vctrs#vctrs>), so here I’ll try to motivate
> vctrs as succinctly as possible by comparing `base::c()` to its
> equivalent `vctrs::vec_c()`. I think the drawbacks of `c()` are well
> known, but to refresh your memory, I’ve highlighted a few at
> <https://github.com/r-lib/vctrs#compared-to-base-r>. I think they arise
> because of two main challenges: `c()` has to both combine vectors *and*
> strip attributes, and it only dispatches on the first argument.
>
> The design of vctrs is largely driven by a pair of principles:
>
> -   The type of `vec_c(x, y)` should be the same as `vec_c(y, x)`
>
> -   The type of `vec_c(x, vec_c(y, z))` should be the same as
>     `vec_c(vec_c(x, y), z)`
>
> i.e. the type should be associative and commutative. I think these are
> good principles because they makes types simpler to understand and to
> implement.
>
> Method dispatch for `vec_c()` is quite simple because associativity and
> commutativity mean that we can determine the output type only by
> considering a pair of inputs at a time. To this end, vctrs provides
> `vec_type2()` which takes two inputs and returns their common type
> (represented as zero length vector):
>
>     str(vec_type2(integer(), double()))
>     #>  num(0)
>
>     str(vec_type2(factor("a"), factor("b")))
>     #>  Factor w/ 2 levels "a","b":
>

What is the reasoning behind taking the union of the levels here? I'm not
sure that is actually the behavior I would want if I have a vector of
factors and I try to append some new data to it. I might want/ expect to
retain the existing levels and get either NAs or an error if the new data
has (present) levels not in the first data. The behavior as above doesn't
seem in-line with what I understand the purpose of factors to be (explicit
restriction of possible values).

I guess what I'm saying is that while I agree associativity is good for
most things, it doesn't seem like the right behavior to me in the case of
factors.

Also, while we're on factors, what does

vec_type2(factor("a"), "a")

return, character or factor with levels "a"?



>
>     # NB: not all types have a common/unifying type
>     str(vec_type2(Sys.Date(), factor("a")))
>     #> Error: No common type for date and factor
>

Why is this not a list? Do you have the additional restraint that vec_type2
must return the class of one of its operands? If so, what is the
justification of that? Are you not counting list as a "type of vector"?


>
> (`vec_type()` currently implements double dispatch through a combination
> of S3 dispatch and if-else blocks, but this will change to a pure S3
> approach in the near future.)
>
> To find the common type of multiple vectors, we can use `Reduce()`:
>
>     vecs <- list(TRUE, 1:10, 1.5)
>
>     type <- Reduce(vec_type2, vecs)
>     str(type)
>     #>  num(0)
>
> There’s one other piece of the puzzle: casting one vector to another
> type. That’s implemented by `vec_cast()` (which also uses double
> dispatch):
>
>     str(lapply(vecs, vec_cast, to = type))
>     #> List of 3
>     #>  $ : num 1
>     #>  $ : num [1:10] 1 2 3 4 5 6 7 8 9 10
>     #>  $ : num 1.5
>
> All up, this means that we can implement the essence of `vec_c()` in
> only a few lines:
>
>     vec_c2 <- function(...) {
>       args <- list(...)
>       type <- Reduce(vec_type, args)
>
>       cast <- lapply(type, vec_cast, to = type)
>       unlist(cast, recurse = FALSE)
>     }
>
>     vec_c(factor("a"), factor("b"))
>     #> [1] a b
>     #> Levels: a b
>
>     vec_c(Sys.Date(), Sys.time())
>     #> [1] "2018-08-06 00:00:00 CDT" "2018-08-06 11:20:32 CDT"
>
> (The real implementation is little more complex:
> <https://github.com/r-lib/vctrs/blob/master/R/c.R>)
>
> On top of this foundation, vctrs expands in a few different ways:
>
> -   To consider the “type” of a data frame, and what the common type of
>     two data frames should be. This leads to a natural implementation of
>     `vec_rbind()` which includes all columns that appear in any input.
>

I must admit I'm a bit surprised here. rbind is one of the few places that
immediately come to mind where R takes a fail early and loud approach to
likely errors (as opposed to the more permissive do soemthing  that could
be what they meant appraoch of, e.g., out-of-bounds indexing). Are we sure
we want rbind to get less strict with respect to compatibility of the
data.frames being combined? Another "permissive" option would be to return
a data.frame which has only the intersection of the columns. There are
certainly times when that is what I want (rather than columns with tons of
NAs in them) and it would be convenient not to need to do the column
subsetting myself. This behavior would also meet your design goals of
associativity and commutivity.

I want to be clear, I think what you describe is a useful operation, if it
is what is intended, but perhaps a different name rather than calling it
rbind? maybe vec_rcbind to indicate that both rows and columns are being
potentially added to any given individual input.

Best,
~G


> -   To create a new “list\_of” type, a list where every element is of
>     fixed type (enforced by `[<-`, `[[<-`, and `$<-`)
>
> -   To think a little about the “shape” of a vector, and to consider
>     recycling as part of the type system. (This thinking is not yet
>     fully fleshed out)
>
> Thanks for making it to the bottom of this long email :) I would love to
> hear your thoughts on vctrs. It’s something that I’ve been having a lot
> of fun exploring, and I’d like to make sure it is as robust as possible
> (and the motivations are as clear as possible) before we start using it
> in other packages.
>
> Hadley
>
>
> --
> http://hadley.nz
>
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



-- 
Gabriel Becker, Ph.D
Scientist
Bioinformatics and Computational Biology
Genentech Research

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