Implement LazyFrame$schema

[Sicheng-Pan]

This PR aims to implement schema, columns, and dtypes for LazyFrame, as requested by #243

[Sicheng-Pan]

@sorhawell It seems that the assignment symbol is changed from = to <- somehow in the wrapper generated by extendr. Any ideas how to configure this so that we can keep using = for assignment?

[sorhawell]

@sorhawell It seems that the assignment symbol is changed from = to <- somehow in the wrapper generated by extendr. Any ideas how to configure this so that we can keep using = for assignment?

my bad, I auto-styled all files in your previous PR, but extendr-wrappers.R should not have been styled.

#251 should revert this, feel free to overrride = immediately

[sorhawell]

@Sicheng-Pan I recommend that you work and submit PRs from individual feature branches in your fork. I got this advice from the rlang maintainer. It will make it easier for you to work on simultanous PRs and a reviewer like me will not mess up your main branch (and all branches thereof) by pushing changes to your PR.

[sorhawell]

looks good :)
I changed to derive columns and dtypes from internal wrapper to show "in $columns" and not "in $schema". It should also be possible to derive from a public method but then something like this pattern should be used

  self$schema |> 
    result() |> 
    unwrap("in $column():") |>
    names()

however currently then err will have two times the same call-info + two times a "in $xxx" info. This is annoying. Currently alot of polars methods are derived from pl$col and pl$lit and the error messages look messy. Before with string error, it was not simple to correct.

I was contemplating to let RPolarsErr have this type

struct RPolarsErr {
  ctx_chain: VecDeque<Rctx>,
  call: Option<Rctx>,
  wherein: Option<Rctx>
}

then any error can only have one call and one wherein. When deriving from another public method, the call and wherein will just be overwritten if an error.

[sorhawell]

@vincentarelbundock this will allow us to bring back names s3 method for LazyFrame without a collect and performance drop :)

[sorhawell]

Thanks @Sicheng-Pan

[vincentarelbundock]

@vincentarelbundock this will allow us to bring back names s3 method for LazyFrame without a collect and performance drop :)

This is awesome! I haven't time to do anything about this recently, and I probably won't have more time in the next couple months. Will probably check in then to see what's to do. Sorry...