[Rd] `merge()` not consistent in how it treats list columns

[Gabriel Becker]

Hi Antoine,


On Sat, Jan 2, 2021 at 11:16 AM Antoine Fabri <antoine.fabri using gmail.com>
wrote:

> Dear R-devel,
>
> When trying to merge 2 data frames by an "id" column, with this column a
> character in one of them, and a list of character in the other, merge
> behaves differently depending which is given first.
>
> Example :
>
> ```
> df1 <- data.frame(a=1)
> df2 <- data.frame(b=2)
> df1$id <- "ID"
> df2$id <- list("ID")
>
> # these print in a similar way, so the upcoming error will be hard to
> diagnose
> df1
> #>   a id
> #> 1 1 ID
> df2
> #>   b id
> #> 1 2 ID
>
> # especially as this works well, df2$id is treated as an atomic vector
> merge(df1, df2)
> #>   id a b
> #> 1 ID 1 2
>

Well, sure but that is because it happens to be a list with each element
having length one. In which case, it really should not have been a list at
all, and the fact that it was seems a deeper problem that should likely be
resolved instead of treating the symptom, in my opinion.

 > df1 <- data.frame(a=1)

> df2 <- data.frame(b=2)

> df1$id <- "ID"

> df2$id <- list(c("ID", "ID2"))

> merge(df1, df2)

[1] id a  b

<0 rows> (or 0-length row.names)


Thats probably not what you wanted it to do, right? Or maybe it is, it
depends, right?. And therein lies the rub.


I have to be honest, as a developer, I really wish this, even in your
example case, threw an error. Anything else just looks to me like a
debugging nightmare looming in the wings waiting to strike.





> # But this fails with a cryptic error message
> merge(df2, df1)
> #> Error in sort.list(bx[m$xi]): 'x' must be atomic for 'sort.list', method
> "shell" and "quick"
> #> Have you called 'sort' on a list?
> ```
>
> I believe that if we let it work one way it should work the other, and that
> if it works neither an explicit error  mentioning how we can't join by list
> column would be helpful.
>

There's no reason (in principle) you wouldn't be able to join by a list
column, they should just both have to be list columns, in my ideal (but
admittedly unlikely) world.  Id rather the atomic-vector/list mismatch case
throw an error, myself.


Now I kind of doubt we can change the behavior that works now, but as Avi
points out, I think this is something that is complicated and case specific
enough that it really ought to be your job as the coder to take care of
what should happen when you try to merge on columns that are fundamentally
different types.


Plus, having an id column as a list, unless it was really explicitly
intentional, seems very likely to be a bug to me. (I mean id column in the
you want to use it to merge things, not the fact that itw as called "id",
though admittedly those are likely to go together...

Best,
~G


>
> Many thanks and happy new year to all the R community,
>
> Antoine
>
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