rlang 0.2.0

This release of rlang is mostly an effort at polishing the tidy evaluation framework. All tidy eval functions and operators have been rewritten in C in order to improve performance. Capture of expression, quasiquotation, and evaluation of quosures are now vastly faster. On the UI side, many of the inconveniences that affected the first release of rlang have been solved:

  • The !! operator now has the precedence of unary + and - which allows a much more natural syntax: !!a > b only unquotes a rather than the whole a > b expression.

  • enquo() works in magrittr pipes: mtcars %>% select(!!enquo(var)).

  • enquos() is a variant of quos() that has a more natural interface for capturing multiple arguments and ....

See the first section below for a complete list of changes to the tidy evaluation framework.

This release also polishes the rlang API. Many functions have been renamed as we get a better feel for the consistency and clarity of the API. Note that rlang as a whole is still maturing and some functions are even experimental. In order to make things clearer for users of rlang, we have started to develop a set of conventions to document the current stability of each function. You will now find “lifecycle” sections in documentation topics. In addition we have gathered all lifecycle information in the ?rlang::lifecycle help page. Please only use functions marked as stable in your projects unless you are prepared to deal with occasional backward incompatible updates.

Tidy evaluation

  • The backend for quos(), exprs(), list2(), dots_list(), etc is now written in C. This greatly improve the performance of dots capture, especially with the splicing operator !!! which now scales much better (you’ll see a 1000x performance gain in some cases). The unquoting algorithm has also been improved which makes enexpr() and enquo() more efficient as well.

  • The tidy eval !! operator now binds tightly. You no longer have to wrap it in parentheses, i.e. !!x > y will only unquote x.

Technically the !! operator has the same precedence as unary - and +. This means that !!a:b and !!a + b are equivalent to (!!a):b and (!!a) + b. On the other hand !!a^b and !!a$b are equivalent to!!(a^b) and !!(a$b).

  • The print method for quosures has been greatly improved. Quosures no longer appear as formulas but as expressions prefixed with ^; quosures are colourised according to their environment; unquoted objects are displayed between angular brackets instead of code (i.e. an unquoted integer vector is shown as <int: 1, 2> rather than 1:2); unquoted S3 objects are displayed using pillar::type_sum() if available.

  • New enquos() function to capture arguments. It treats ... the same way as quos() but can also capture named arguments just like enquo(), i.e. one level up. By comparison quos(arg) only captures the name arg rather than the expression supplied to the arg argument.

In addition, enexprs() is like enquos() but like exprs() it returns bare expressions. And ensyms() expects strings or symbols.

  • It is now possible to use enquo() within a magrittr pipe:

select_one <- function(df, var) { df %>% dplyr::select(!!enquo(var)) }

Technically, this is because enquo() now also captures arguments in parents of the current environment rather than just in the current environment. The flip side of this increased flexibility is that if you made a typo in the name of the variable you want to capture, and if an object of that name exists anywhere in the parent contexts, you will capture that object rather than getting an error.

  • quo_expr() has been renamed to quo_squash() in order to better reflect that it is a lossy operation that flattens all nested quosures.

  • !!! now accepts any kind of objects for consistency. Scalar types are treated as vectors of length 1. Previously only symbolic objects like symbols and calls were treated as such.

  • ensym() is a new variant of enexpr() that expects a symbol or a string and always returns a symbol. If a complex expression is supplied it fails with an error.

  • exprs() and quos() gain a .unquote_names arguments to switch off interpretation of := as a name operator. This should be useful for programming on the language targetting APIs such as data.table.

  • exprs() gains a .named option to auto-label its arguments (#267).

  • Functions taking dots by value rather than by expression (e.g. regular functions, not quoting functions) have a more restricted set of unquoting operations. They only support := and !!!, and only at top-level. I.e. dots_list(!!! x) is valid but not dots_list(nested_call(!!! x)) (#217).

  • Functions taking dots with list2() or dots_list() now support splicing of NULL values. !!! NULL is equivalent to !!! list() (#242).

  • Capture operators now support evaluated arguments. Capturing a forced or evaluated argument is exactly the same as unquoting that argument: the actual object (even if a vector) is inlined in the expression. Capturing a forced argument occurs when you use enquo(), enexpr(), etc too late. It also happens when your quoting function is supplied to lapply() or when you try to quote the first argument of an S3 method (which is necessarily evaluated in order to detect which class to dispatch to). (#295, #300).

  • Parentheses around !! are automatically removed. This makes the generated expression call cleaner: (!! sym("name"))(arg). Note that removing the parentheses will never affect the actual precedence within the expression as the parentheses are only useful when parsing code as text. The parentheses will also be added by R when printing code if needed (#296).

  • Quasiquotation now supports !! and !!! as functional forms:

expr(`!!`(var)) quo(call(`!!!`(var)))

This is consistent with the way native R operators parses to function calls. These new functional forms are to be preferred to UQ() and UQS(). We are now questioning the latter and might deprecate them in a future release.


  • The new functions cnd_warn() and cnd_inform() transform conditions to warnings or messages before signalling them.

  • cnd_signal() now returns invisibly.

  • cnd_signal() and cnd_abort() now accept character vectors to create typed conditions with several S3 subclasses.

  • is_condition() is now properly exported.

  • Condition signallers such as cnd_signal() and abort() now accept a call depth as call arguments. This allows plucking a call from further up the call stack (#30).

  • New helper catch_cnd(). This is a small wrapper around tryCatch() that captures and returns any signalled condition. It returns NULL if none was signalled.

  • cnd_abort() now adds the correct S3 classes for error conditions. This fixes error catching, for instance by testthat::expect_error().


Various features

  • New functions inherits_any(), inherits_all(), and inherits_only(). They allow testing for inheritance from multiple classes. The _any variant is equivalent to base::inherits() but is more explicit about its behaviour. inherits_all() checks that all classes are present in order and inherits_only() checks that the class vectors are identical.

  • New fn_fmls<- and fn_fmls_names<- setters.

  • New function experimental function chr_unserialise_unicode() for turning characters serialised to unicode point form (e.g. <U+xxxx>) to UTF-8. In addition, as_utf8_character() now translates those as well. (@krlmlr)

  • expr_label() now supports quoted function definition calls (#275).

  • call_modify() and call_standardise() gain an argument to specify an environment. The call definition is looked up in that environment when the call to modify or standardise is not wrapped in a quosure.

  • is_symbol() gains a name argument to check that that the symbol name matches a string (#287).

  • New rlang_box class. Its purpose is similar to the AsIs class from base::I(), i.e. it protects a value temporarily. However it does so by wrapping the value in a scalar list. Use new_box() to create a boxed value, is_box() to test for a boxed value, and unbox() to unbox it. new_box() and is_box() accept optional subclass.

  • The vector constructors such as new_integer(), new_double_along() etc gain a names argument. In the case of the _along family it defaults to the names of the input vector.


  • When nested quosures are evaluated with eval_tidy(), the .env pronoun now correctly refers to the current quosure under evaluation (#174). Previously it would always refer to the environment of the outermost quosure.

  • as_pairlist() (part of the experimental API) now supports NULL and objects of type pairlist (#397).

  • Fixed a performance bug in set_names() that caused a full copy of the vector names (@jimhester, #366).

API changes

The rlang API is maturing and still in flux. However we have made an effort to better communicate what parts are stable. We will not introduce breaking changes for stable functions unless the payoff for the change is worth the trouble. See ?rlang::lifecycle for the lifecycle status of exported functions.

In rlang 0.1 calls were called “language” objects in order to follow the R type nomenclature as returned by base::typeof(). We wanted to avoid adding to the confusion between S modes and R types. With hindsight we find it is better to use more meaningful type names.

  • We now use the term “data mask” instead of “overscope”. We think data mask is a more natural name in the context of R. We say that that objects from user data mask objects in the current environment. This makes reference to object masking in the search path which is due to the same mechanism (in technical terms, lexical scoping with hierarchically nested environments).

Following this new terminology, the new functions as_data_mask() and new_data_mask() replace as_overscope() and new_overscope(). as_data_mask() has also a more consistent interface. These functions are only meant for developers of tidy evaluation interfaces.

  • We no longer require a data mask (previously called overscope) to be cleaned up after evaluation. overscope_clean() is thus soft-deprecated without replacement.

Breaking changes

  • !! now binds tightly in order to match intuitive parsing of tidy eval code, e.g. !! x > y is now equivalent to (!! x) > y. A corollary of this new syntax is that you now have to be explicit when you want to unquote the whole expression on the right of !!. For instance you have to explicitly write !! (x > y) to unquote x > y rather than just x.

  • UQ(), UQS() and := now issue an error when called directly. The previous definitions caused surprising results when the operators were invoked in wrong places (i.e. not in quasiquoted arguments).

  • The prefix form `!!`() is now an alias to !! rather than UQE(). This makes it more in line with regular R syntax where operators are parsed as regular calls, e.g. a + b is parsed as `+`(a, b) and both forms are completely equivalent. Also the prefix form `!!!`() is now equivalent to !!!.

  • UQE() is now deprecated in order to simplify the syntax of quasiquotation. Please use !! get_expr(x) instead.

  • expr_interp() now returns a formula instead of a quosure when supplied a formula.

  • is_quosureish() and as_quosureish() are deprecated. These functions assumed that quosures are formulas but that is only an implementation detail.

  • new_cnd() is now cnd() for consistency with other constructors. Also, cnd_error(), cnd_warning() and cnd_message() are now error_cnd(), warning_cnd() and message_cnd() to follow our naming scheme according to which the type of output is a suffix rather than a prefix.

  • is_node() now returns TRUE for calls as well and is_pairlist() does not return TRUE for NULL objects. Use is_node_list() to determine whether an object either of type pairlist or NULL. Note that all these functions are still experimental.

  • set_names() no longer automatically splices lists of character vectors as we are moving away from automatic splicing semantics.

Upcoming breaking changes

  • Calling the functional forms of unquote operators with the rlang namespace qualifier is soft-deprecated. UQ() and UQS() are not function calls so it does not make sense to namespace them. Supporting namespace qualifiers complicates the implementation of unquotation and is misleading as to the nature of unquoting (which are syntactic operators at quotation-time rather than function calls at evaluation-time).

  • We are now questioning UQ() and UQS() as functional forms of !!. If !! and !!! were native R operators, they would parse to the functional calls `!!`() and `!!!`(). This is now the preferred way to unquote with a function call rather than with the operators. We haven’t decided yet whether we will deprecate UQ() and UQS() in the future. In any case we recommend using the new functional forms.

  • parse_quosure() and parse_quosures() are soft-deprecated in favour of parse_quo() and parse_quos(). These new names are consistent with the rule that abbreviated suffixes indicate the return type of a function. In addition the new functions require their callers to explicitly supply an environment for the quosures.

  • Using f_rhs() and f_env() on quosures is soft-deprecated. The fact that quosures are formulas is an implementation detail that might change in the future. Please use quo_get_expr() and quo_get_env() instead.

  • quo_expr() is soft-deprecated in favour of quo_squash(). quo_expr() was a misnomer because it implied that it was a mere expression acccessor for quosures whereas it was really a lossy operation that squashed all nested quosures.

  • With the renaming of the lang particle to call, all these functions are soft-deprecated: lang(), is_lang(), lang_fn(), lang_name(), lang_args().

In addition, lang_head() and lang_tail() are soft-deprecated without replacement because these are low level accessors that are rarely needed.

  • as_overscope() is soft-deprecated in favour of as_data_mask().

  • The node setters were renamed from mut_node_ prefix to node_poke_. This change follows a new naming convention in rlang where mutation is referred to as “poking”.

  • splice() is now in questioning stage as it is not needed given the !!! operator works in functions taking dots with dots_list().

  • lgl_len(), int_len() etc have been soft-deprecated and renamed with new_ prefix, e.g. new_logical() and new_integer(). This is for consistency with other non-variadic object constructors.

  • ll() is now an alias to list2(). This is consistent with the new call2() constructor for calls. list2() and call2() are versions of list() and call() that support splicing of lists with !!!. ll() remains around as a shorthand for users who like its conciseness.

  • Automatic splicing of lists in vector constructors (e.g. lgl(), chr(), etc) is now soft-deprecated. Please be explicit with the splicing operator !!!.

rlang 0.1.6

  • This is a maintenance release in anticipation of a forthcoming change to R’s C API (use MARK_NOT_MUTABLE() instead of SET_NAMED()).

  • New function is_reference() to check whether two objects are one and the same.

rlang 0.1.4

  • eval_tidy() no longer maps over lists but returns them literally. This behaviour is an overlook from past refactorings and was never documented.

rlang 0.1.2

This hotfix release makes rlang compatible with the R 3.1 branch.

rlang 0.1.1

This release includes two important fixes for tidy evaluation:

  • Bare formulas are now evaluated in the correct environment in tidyeval functions.

  • enquo() now works properly within compiled functions. Before this release, constants optimised by the bytecode compiler couldn’t be enquoted.

New functions:

  • The new_environment() constructor creates a child of the empty environment and takes an optional named list of data to populate it. Compared to env() and child_env(), it is meant to create environments as data structures rather than as part of a scope hierarchy.

  • The new_call() constructor creates calls out of a callable object (a function or an expression) and a pairlist of arguments. It is useful to avoid costly internal coercions between lists and pairlists of arguments.

UI improvements:

  • env_child()’s first argument is now .parent instead of parent.

  • mut_ setters like mut_attrs() and environment helpers like env_bind() and env_unbind() now return their (modified) input invisibly. This follows the tidyverse convention that functions called primarily for their side effects should return their input invisibly.

  • is_pairlist() now returns TRUE for NULL. We added is_node() to test for actual pairlist nodes. In other words, is_pairlist() tests for the data structure while is_node() tests for the type.


  • env() and env_child() can now get arguments whose names start with .. Prior to this fix, these arguments were partial-matching on env_bind()’s .env argument.

  • The internal replace_na() symbol was renamed to avoid a collision with an exported function in tidyverse. This solves an issue occurring in old versions of R prior to 3.3.2 (#133).

rlang 0.1.0

Initial release.