rlang 0.4.0

Tidy evaluation

Interpolate function inputs with the curly-curly operator

The main change of this release is the new tidy evaluation operator
{{. This operator abstracts the quote-and-unquote idiom into a
single interpolation step:

my_wrapper <- function(data, var, by) {
  data %>%
    group_by({{ by }}) %>%
    summarise(average = mean({{ var }}, na.rm = TRUE))
}

{{ var }} is a shortcut for !!enquo(var) that should be easier on
the eyes, and easier to learn and teach.

Note that for multiple inputs, the existing documentation doesn't
stress enough that you can just pass dots straight to other tidy eval
functions. There is no need for quote-and-unquote unless you need to
modify the inputs or their names in some way:

my_wrapper <- function(data, var, ...) {
  data %>%
    group_by(...) %>%
    summarise(average = mean({{ var }}, na.rm = TRUE))
}

More robust .env pronoun

Another improvement to tidy evaluation should make it easier to use
the .env pronoun. Starting from this release, subsetting an object
from the .env pronoun now evaluates the corresponding symbol. This
makes .env more robust, in particular in magrittr pipelines. The
following example would previously fail:

foo <- 10
mtcars %>% mutate(cyl = cyl * .env$foo)

This way, using the .env pronoun is now equivalent to unquoting a
constant objects, but with an easier syntax:

mtcars %>% mutate(cyl = cyl * !!foo)

Note that following this change, and despite its name, .env is no
longer referring to a bare environment. Instead, it is a special
shortcut with its own rules. Similarly, the .data pronoun is not
really a data frame.

New functions and features

• New pairlist2() function with splicing support. It preserves
  missing arguments, which makes it useful for lists of formal
  parameters for functions.

• is_bool() is a scalar type predicate that checks whether its input
  is a single TRUE or FALSE. Like is_string(), it returns
  FALSE when the input is missing. This is useful for type-checking
  function arguments (#695).

• is_string() gains a string argument. is_string(x, "foo") is a
  shortcut for is_character(x) && length(x) == 1 && identical(x, "foo").

• Lists of quosures now have pillar methods for display in tibbles.

• set_names() now names unnamed input vectors before applying a
  function. The following expressions are now equivalent:

  letters %>% set_names() %>% set_names(toupper)
  
  letters %>% set_names(toupper)
  

• You can now pass a character vector as message argument for
  abort(), warn(), inform(), and signal(). The vector is
  collapsed to a single string with a "\n" newline separating each
  element of the input vector (#744).

• maybe_missing() gains a default argument.

• New functions for weak references: new_weakref(), weakref_key(),
  weakref_value(), and is_weakref() (@wch, #787).

Performance

• The performance of exec() has been improved. It is now on the same
  order of performance as do.call(), though slightly slower.

• call2() now uses the new pairlist2() function internally. This
  considerably improves its performance. This also means it now
  preserves empty arguments:

  call2("fn", 1, , foo = )
  #> fn(1, , foo = )
  

Bugfixes and small improvements

• with_handlers() now installs calling handlers first on the stack,
  no matter their location in the argument list. This way they always
  take precedence over exiting handlers, which ensures their side
  effects (such as logging) take place (#718).

• In rlang backtraces, the global:: prefix is now only added when
  the function directly inherits from the global environment.
  Functions inheriting indirectly no longer have a namespace
  qualifier (#733).

• options(error = rlang::entrace) now has better support for errors
  thrown from C (#779). It also saves structured errors in the error
  field of rlang::last_error().

• ns_env() and ns_env_name() (experimental functions) now support
  functions and environments consisently. They also require an
  argument from now on.

• is_interactive() is aware of the TESTTHAT environment variable and
  returns FALSE when it is "true" (@jennybc, #738).

• fn_fmls() and variants no longer coerce their input to a
  closure. Instead, they throw an error.

• Fixed an issue in knitr that caused backtraces to print even when error = TRUE.

• The return object from as_function() now inherits from
  "function" (@richierocks, #735).

Lifecycle

We commit to support 5 versions of R. As R 3.6 is about to be
released, rlang now requires R 3.2 or greater. We're also continuing
our efforts to streamline and narrow the rlang API.

• modify() and prepend() (two experimental functions marked as in
  the questioning stage since rlang 0.3.0) are now deprecated. Vector
  functions are now out of scope for rlang. They might be revived in
  the vctrs or funs packages.

• exiting() is soft-deprecated because with_handlers() treats
  handlers as exiting by default.

• The vector constructors like lgl() or new_logical() are now in
  the questioning stage. They are likely to be moved to the vctrs
  package at some point. Same for the missing values shortcuts like
  na_lgl.

• as_logical(), as_integer(), etc have been soft-deprecated in
  favour of vctrs::vec_cast().

• type_of(), switch_type(), coerce_type(), and friends are
  soft-deprecated.

• The encoding and locale API was summarily archived. This API didn't
  bring any value and wasn't used on CRAN.

• lang_type_of(), switch_lang(), and coerce_lang() were
  archived. These functions were not used on CRAN or internally.

• Subsetting quosures with [ or [[ is soft-deprecated.

• All functions that were soft-deprecated, deprecated, or defunct in
  previous releases have been bumped to the next lifecycle stage.