tidyeval {wikkitidy}

Tidy eval helpers

Description

This page lists the tidy eval tools reexported in this package from rlang. To learn about using tidy eval in scripts and packages at a high level, see the dplyr programming vignette and the ggplot2 in packages vignette. The Metaprogramming section of Advanced R may also be useful for a deeper dive.

• The tidy eval operators ⁠{{⁠, ⁠!!⁠, and ⁠!!!⁠ are syntactic constructs which are specially interpreted by tidy eval functions. You will mostly need ⁠{{⁠, as ⁠!!⁠ and ⁠!!!⁠ are more advanced operators which you should not have to use in simple cases.

  The curly-curly operator ⁠{{⁠ allows you to tunnel data-variables passed from function arguments inside other tidy eval functions. ⁠{{⁠ is designed for individual arguments. To pass multiple arguments contained in dots, use ... in the normal way.

  my_function <- function(data, var, ...) {
    data %>%
      group_by(...) %>%
      summarise(mean = mean({{ var }}))
  }
  

• enquo() and enquos() delay the execution of one or several function arguments. The former returns a single expression, the latter returns a list of expressions. Once defused, expressions will no longer evaluate on their own. They must be injected back into an evaluation context with ⁠!!⁠ (for a single expression) and ⁠!!!⁠ (for a list of expressions).

  my_function <- function(data, var, ...) {
    # Defuse
    var <- enquo(var)
    dots <- enquos(...)
  
    # Inject
    data %>%
      group_by(!!!dots) %>%
      summarise(mean = mean(!!var))
  }
  

  In this simple case, the code is equivalent to the usage of ⁠{{⁠ and ... above. Defusing with enquo() or enquos() is only needed in more complex cases, for instance if you need to inspect or modify the expressions in some way.

• The .data pronoun is an object that represents the current slice of data. If you have a variable name in a string, use the .data pronoun to subset that variable with [[.

  my_var <- "disp"
  mtcars %>% summarise(mean = mean(.data[[my_var]]))
  

• Another tidy eval operator is ⁠:=⁠. It makes it possible to use glue and curly-curly syntax on the LHS of =. For technical reasons, the R language doesn't support complex expressions on the left of =, so we use ⁠:=⁠ as a workaround.

  my_function <- function(data, var, suffix = "foo") {
    # Use `{{` to tunnel function arguments and the usual glue
    # operator `{` to interpolate plain strings.
    data %>%
      summarise("{{ var }}_mean_{suffix}" := mean({{ var }}))
  }
  

• Many tidy eval functions like dplyr::mutate() or dplyr::summarise() give an automatic name to unnamed inputs. If you need to create the same sort of automatic names by yourself, use as_label(). For instance, the glue-tunnelling syntax above can be reproduced manually with:

  my_function <- function(data, var, suffix = "foo") {
    var <- enquo(var)
    prefix <- as_label(var)
    data %>%
      summarise("{prefix}_mean_{suffix}" := mean(!!var))
  }
  

  Expressions defused with enquo() (or tunnelled with ⁠{{⁠) need not be simple column names, they can be arbitrarily complex. as_label() handles those cases gracefully. If your code assumes a simple column name, use as_name() instead. This is safer because it throws an error if the input is not a name as expected.

Value

Consult the original rlang documentation for the return types of these re-exported functions.