R and Polars expressions

When we use the tidyverse, we use R expressions in mainly three places: filter(), mutate(), and summarize().

#> Error in get(paste0(generic, ".", class), envir = get_method_env()) : 
#>   object 'type_sum.accel' not found

library(dplyr, warn.conflicts = FALSE)

filter(mtcars, am + gear > carb)
mutate(mtcars, x = (qsec - mean(qsec)) / sd(qsec))
mtcars |>
  group_by(cyl) |>
  summarize(x = mean(qsec) / sd(qsec))

This is very convenient but creates a challenge for tidypolars. Indeed, while it is possible to pass R functions directly to a Polars Data/LazyFrame, it is strongly discouraged to do so because it doesn’t take advantage of Polars optimizations.

Indeed, Polars comes with dozens of built-in functions for maths (median, var, arccos, …), string manipulation (len_chars, starts, …), and date-time (hour, quarter, ordinal_day, …). All of these functions are optimized internally and are ran in parallel under the hood, which will not be the case if we pass R functions.

However, using these Polars expressions would imply that we need to learn these new functions and this new syntax. To avoid doing that, tidypolars will automatically translate R expressions into Polars ones. Basically, you can keep writing R expressions in most situations, and they will automatically be translated to Polars syntax.

However, there are some situations where this might not work, so this vignette explains the process and the limitations.

How does tidypolars translate R expressions into Polars expressions?

When tidypolars receives an expression, it runs a function translate() several times until all components are translated to their Polars equivalent. There are four possible components: single values, column names, external objects, and functions.

Single values, column names, and external objects

If you pass a single value, like x = 1 or x = "a", it is wrapped into pl$lit(). This is also the case for external objects with the difference that these need to be wrapped in {{ }} and are evaluated before being wrapped into pl$lit().

Column names, like x = mpg, are wrapped into pl$col().

x = "a"               ->  x = pl$lit("a")
x = {{ some_value }}  ->  x = pl$lit(*value*)
x = mpg               ->  x = pl$col("mpg")

Functions

Functions are split into two categories: built-in functions (i.e functions provided by base R or by other packages), and user-defined functions (UDF) that are written by the user (you).

Built-in functions

In the first case, tidypolars checks the function name and whether it has already been translated internally. For example, if we call the R function mean(x, trim = 2), then it looks for a translation of mean(). You can see the list of supported R functions at the bottom of this vignette. Note that most of essential base R functions are supported, as well as many functions from dplyr or from stringr for example.

Now that tidypolars knows that a translation of mean() exists, it parses the arguments in the call to translate them to the Polars syntax: internally, x is converted to pl$col("x") if there is a column "x" in the data. Sometimes, additional arguments do not have an equivalent in Polars. This is the case for the argument trim here. In this case, tidypolars ignores this argument and warns the user:

library(tidypolars)
library(polars)

mtcars |>
  as_polars_df() |>
  mutate(x = mean(mpg, trim = 2))
#> Warning: 
#> Package tidypolars doesn't know how to use some arguments of `mean()`.
#> The following argument(s) will be ignored: `trim`.
#> shape: (32, 12)
#> ┌──────┬─────┬───────┬───────┬───┬─────┬──────┬──────┬───────────┐
#> │ mpg  ┆ cyl ┆ disp  ┆ hp    ┆ … ┆ am  ┆ gear ┆ carb ┆ x         │
#> │ ---  ┆ --- ┆ ---   ┆ ---   ┆   ┆ --- ┆ ---  ┆ ---  ┆ ---       │
#> │ f64  ┆ f64 ┆ f64   ┆ f64   ┆   ┆ f64 ┆ f64  ┆ f64  ┆ f64       │
#> ╞══════╪═════╪═══════╪═══════╪═══╪═════╪══════╪══════╪═══════════╡
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 20.090625 │
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 20.090625 │
#> │ 22.8 ┆ 4.0 ┆ 108.0 ┆ 93.0  ┆ … ┆ 1.0 ┆ 4.0  ┆ 1.0  ┆ 20.090625 │
#> │ 21.4 ┆ 6.0 ┆ 258.0 ┆ 110.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 1.0  ┆ 20.090625 │
#> │ 18.7 ┆ 8.0 ┆ 360.0 ┆ 175.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 2.0  ┆ 20.090625 │
#> │ …    ┆ …   ┆ …     ┆ …     ┆ … ┆ …   ┆ …    ┆ …    ┆ …         │
#> │ 30.4 ┆ 4.0 ┆ 95.1  ┆ 113.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 2.0  ┆ 20.090625 │
#> │ 15.8 ┆ 8.0 ┆ 351.0 ┆ 264.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 4.0  ┆ 20.090625 │
#> │ 19.7 ┆ 6.0 ┆ 145.0 ┆ 175.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 6.0  ┆ 20.090625 │
#> │ 15.0 ┆ 8.0 ┆ 301.0 ┆ 335.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 8.0  ┆ 20.090625 │
#> │ 21.4 ┆ 4.0 ┆ 121.0 ┆ 109.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 2.0  ┆ 20.090625 │
#> └──────┴─────┴───────┴───────┴───┴─────┴──────┴──────┴───────────┘

This behavior can be changed to throw an error instead.

User-defined functions

User-defined functions (UDF) are more challenging. Indeed, it is technically possible to inspect the code inside a UDF, but rewriting it to match Polars syntax would be extremely complicated. In this situation, you will have to rewrite your custom function using Polars syntax so that it returns a Polars expression. For example, we could make a function to standardize a column like this:

pl_standardize <- function(x) {
  (x - x$mean()) / x$std()
}

Remember that the column name used as x will end up wrapped into pl$col(), so to check that your function returns a Polars expression, you have to provide a pl$col() call:

pl_standardize(pl$col("mpg"))
#> polars Expr: [([(col("mpg")) - (col("mpg").mean())]) // (col("mpg").std())]

This function correctly returns a Polars expression, so we can now use it like any other function:

mtcars |>
  as_polars_df() |>
  mutate(x = pl_standardize(mpg))
#> shape: (32, 12)
#> ┌──────┬─────┬───────┬───────┬───┬─────┬──────┬──────┬───────────┐
#> │ mpg  ┆ cyl ┆ disp  ┆ hp    ┆ … ┆ am  ┆ gear ┆ carb ┆ x         │
#> │ ---  ┆ --- ┆ ---   ┆ ---   ┆   ┆ --- ┆ ---  ┆ ---  ┆ ---       │
#> │ f64  ┆ f64 ┆ f64   ┆ f64   ┆   ┆ f64 ┆ f64  ┆ f64  ┆ f64       │
#> ╞══════╪═════╪═══════╪═══════╪═══╪═════╪══════╪══════╪═══════════╡
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 0.150885  │
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 0.150885  │
#> │ 22.8 ┆ 4.0 ┆ 108.0 ┆ 93.0  ┆ … ┆ 1.0 ┆ 4.0  ┆ 1.0  ┆ 0.449543  │
#> │ 21.4 ┆ 6.0 ┆ 258.0 ┆ 110.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 1.0  ┆ 0.217253  │
#> │ 18.7 ┆ 8.0 ┆ 360.0 ┆ 175.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 2.0  ┆ -0.230735 │
#> │ …    ┆ …   ┆ …     ┆ …     ┆ … ┆ …   ┆ …    ┆ …    ┆ …         │
#> │ 30.4 ┆ 4.0 ┆ 95.1  ┆ 113.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 2.0  ┆ 1.710547  │
#> │ 15.8 ┆ 8.0 ┆ 351.0 ┆ 264.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 4.0  ┆ -0.711907 │
#> │ 19.7 ┆ 6.0 ┆ 145.0 ┆ 175.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 6.0  ┆ -0.064813 │
#> │ 15.0 ┆ 8.0 ┆ 301.0 ┆ 335.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 8.0  ┆ -0.844644 │
#> │ 21.4 ┆ 4.0 ┆ 121.0 ┆ 109.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 2.0  ┆ 0.217253  │
#> └──────┴─────┴───────┴───────┴───┴─────┴──────┴──────┴───────────┘

Special case: across()

across() is a very useful function that applies a function (or a list of functions) to a selection of columns. It accepts built-in functions, UDFs, and anonymous functions.

mtcars |>
  as_polars_df() |>
  mutate(
    across(
      .cols = contains("a"),
      list(mean = mean, stand = pl_standardize, ~ sd(.x))
    )
  )
#> shape: (32, 23)
#> ┌──────┬─────┬───────┬───────┬───┬──────────┬───────────┬────────────┬────────┐
#> │ mpg  ┆ cyl ┆ disp  ┆ hp    ┆ … ┆ gear_3   ┆ carb_mean ┆ carb_stand ┆ carb_3 │
#> │ ---  ┆ --- ┆ ---   ┆ ---   ┆   ┆ ---      ┆ ---       ┆ ---        ┆ ---    │
#> │ f64  ┆ f64 ┆ f64   ┆ f64   ┆   ┆ f64      ┆ f64       ┆ f64        ┆ f64    │
#> ╞══════╪═════╪═══════╪═══════╪═══╪══════════╪═══════════╪════════════╪════════╡
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ 0.735203   ┆ 1.6152 │
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ 0.735203   ┆ 1.6152 │
#> │ 22.8 ┆ 4.0 ┆ 108.0 ┆ 93.0  ┆ … ┆ 0.737804 ┆ 2.8125    ┆ -1.122152  ┆ 1.6152 │
#> │ 21.4 ┆ 6.0 ┆ 258.0 ┆ 110.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ -1.122152  ┆ 1.6152 │
#> │ 18.7 ┆ 8.0 ┆ 360.0 ┆ 175.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ -0.503034  ┆ 1.6152 │
#> │ …    ┆ …   ┆ …     ┆ …     ┆ … ┆ …        ┆ …         ┆ …          ┆ …      │
#> │ 30.4 ┆ 4.0 ┆ 95.1  ┆ 113.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ -0.503034  ┆ 1.6152 │
#> │ 15.8 ┆ 8.0 ┆ 351.0 ┆ 264.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ 0.735203   ┆ 1.6152 │
#> │ 19.7 ┆ 6.0 ┆ 145.0 ┆ 175.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ 1.97344    ┆ 1.6152 │
#> │ 15.0 ┆ 8.0 ┆ 301.0 ┆ 335.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ 3.211677   ┆ 1.6152 │
#> │ 21.4 ┆ 4.0 ┆ 121.0 ┆ 109.0 ┆ … ┆ 0.737804 ┆ 2.8125    ┆ -0.503034  ┆ 1.6152 │
#> └──────┴─────┴───────┴───────┴───┴──────────┴───────────┴────────────┴────────┘

Similarly, UDFs and anonymous functions will error if they don’t return a Polars expression:

mtcars |>
  as_polars_df() |>
  mutate(
    across(
      .cols = contains("a"),
      .fns = list(
        mean = mean,
        function(x) {
          (x - mean(x)) / sd(x)
        },
        ~ sd(.x)
      )
    )
  )
#> Error in `mutate()`:
#> ! Could not evaluate an anonymous function in `across()`.
#> ℹ Are you sure the anonymous function returns a Polars expression?

List of base R and tidyverse functions supported by tidypolars

Package	Function	Notes
base	abs
base	acos
base	acosh
base	all
base	any
base	asin
base	asinh
base	atan
base	atanh
base	ceiling
base	cos
base	cosh
base	cummin
base	cumsum
base	diff
base	exp
base	floor
base	grepl
base	ifelse
base	ISOdatetime
base	length
base	log
base	log10
base	max
base	mean
base	min
base	nchar
base	paste0
base	paste
base	rank
base	rev
base	round
base	sin
base	sinh
base	sort
base	sqrt
base	strptime
base	substr
base	tan
base	tanh
base	tolower
base	toupper
base	unique
base	which.min
base	which.max
dplyr	between
dplyr	case_match
dplyr	case_when
dplyr	coalesce
dplyr	consecutive_id
dplyr	dense_rank
dplyr	first
dplyr	group_keys
dplyr	group_vars
dplyr	if_else
dplyr	lag
dplyr	lead
dplyr	last
dplyr	min_rank
dplyr	n
dplyr	nth
dplyr	n_distinct
dplyr	row_number	Doesn’t work when x is missing.
lubridate	day
lubridate	ddays
lubridate	dhours
lubridate	dmilliseconds
lubridate	dminutes
lubridate	dseconds
lubridate	dweeks
lubridate	make_date
lubridate	make_datetime	In lubridate::make_datetime(), when there is an overflow (for example hours = 25), then it is automatically converted to the higher unit (for example 1 day and 1h). In Polars, this returns NA.
lubridate	mday
lubridate	month
lubridate	quarter
lubridate	wday	Requires week_start == 7. If label = TRUE, it returns a string variable and not a factor as in lubridate.
lubridate	yday
lubridate	year
stats	median
stats	lag
stats	sd
stats	var
stringr	regex
stringr	str_count
stringr	str_detect
stringr	str_dup
stringr	str_ends
stringr	str_extract
stringr	str_extract_all
stringr	str_length
stringr	str_pad
stringr	str_remove
stringr	str_remove_all
stringr	str_replace
stringr	str_replace_all
stringr	str_replace_na
stringr	str_split
stringr	str_split_i
stringr	str_squish
stringr	str_starts
stringr	str_sub
stringr	str_trim
stringr	str_to_lower
stringr	str_to_title	Letters following apostrophe will be capitalized as well, which differs from the stringr implementation.
stringr	str_to_upper
stringr	str_trunc
stringr	word
tidyr	replace_na
tools	toTitleCase	Letters following apostrophe will be capitalized as well, which differs from the tools implementation.