DataFrame$tail()

[vincentarelbundock]

This is a first attempt at a minimal commit. Is this the kind of contribution you would welcome? What should I do different to facilitate a merge?

library(rpolars)

pl$DataFrame(mtcars)$tail(2)
# polars DataFrame: shape: (2, 11)
# ┌──────┬─────┬───────┬───────┬─────┬─────┬─────┬──────┬──────┐
# │ mpg  ┆ cyl ┆ disp  ┆ hp    ┆ ... ┆ vs  ┆ am  ┆ gear ┆ carb │
# │ ---  ┆ --- ┆ ---   ┆ ---   ┆     ┆ --- ┆ --- ┆ ---  ┆ ---  │
# │ f64  ┆ f64 ┆ f64   ┆ f64   ┆     ┆ f64 ┆ f64 ┆ f64  ┆ f64  │
# ╞══════╪═════╪═══════╪═══════╪═════╪═════╪═════╪══════╪══════╡
# │ 15.0 ┆ 8.0 ┆ 301.0 ┆ 335.0 ┆ ... ┆ 0.0 ┆ 1.0 ┆ 5.0  ┆ 8.0  │
# │ 21.4 ┆ 4.0 ┆ 121.0 ┆ 109.0 ┆ ... ┆ 1.0 ┆ 1.0 ┆ 4.0  ┆ 2.0  │
# └──────┴─────┴───────┴───────┴─────┴─────┴─────┴──────┴──────┘

tail(mtcars, 2)
#                mpg cyl disp  hp drat   wt qsec vs am gear carb
# Maserati Bora 15.0   8  301 335 3.54 3.57 14.6  0  1    5    8
# Volvo 142E    21.4   4  121 109 4.11 2.78 18.6  1  1    4    2

[sorhawell]

Hi @vincentarelbundock. Thank you so much for finding time for writing a translation :)

This is pretty alright. I would take the PR as it is right now.

I will add some suggestions in the review, I came to appreciate, when I wrote the later translations.

[vincentarelbundock]

Thanks for the hints. I think the latest commit resolves all outstanding issues.

I still don't quite understand what is going on under the hood, but I'll try to translate a few other functions. Will open a separate PR for those.

[sorhawell]

Thanks for the hints. I think the latest commit resolves all outstanding issues.

I still don't quite understand what is going on under the hood, but I'll try to translate a few other functions.

Error handling in rpolars:
It is not possible to raise R errors from rust side with extendr without bleeding memory. Instead an Result-enum Result<LazyFrame, String> is returned to R side as an R list list(ok=NULL, err = "oups!"). On R side typically the Result is just unwrap()'ed immediately to raise the error.
r_result_list() is the old less ergonomic way to convert Result to R list. Now rpolars use a PR branch of extendr where the conversion to R list will occur implicitly, as you have done here.

robj_to!:
robj_to! is just a macro which do custom conversions from R to rust types + error handling. The vanilla extendr conversions aren't bad, but not always ideal and error messages are very sparse. By using robj_to! all conversions and err handling can be modified throughout the code base at one single place.
robj_to! basically dispatches the appropriate conversion function and do error handling. There is a candidate master student project to upgrade the error handling this summer.

[sorhawell]

many thanks @vincentarelbundock for this excellent contribution :)

[vincentarelbundock]

Thanks for the merge and the info, this is very useful.