In rlang, an expression is the return type of
set of objects that can be obtained from parsing R code. Under this
definition expressions include numbers, strings,
and function calls. These objects can be classified as:
Symbolic objects, i.e. symbols and function calls (for which
Syntactic literals, i.e. scalar atomic objects and
TRUE if the input is either a symbolic
object or a syntactic literal. If a call, the elements of the call
must all be expressions as well. Unparsable calls are not
considered expressions in this narrow definition.
Note that in base R, there exists
expression() vectors, a data
type similar to a list that supports special attributes created by
the parser called source references. This data type is not
supported in rlang.
is_expression(x) is_syntactic_literal(x) is_symbolic(x)
An object to test.
TRUE for symbols and calls (objects with
language). Symbolic objects are replaced by their value
during evaluation. Literals are the complement of symbolic
objects. They are their own value and return themselves during
is_syntactic_literal() is a predicate that returns
TRUE for the
subset of literals that are created by R when parsing text (see
parse_expr()): numbers, strings and
NULL. Along with symbols,
these literals are the terminating nodes in an AST.
Note that in the most general sense, a literal is any R object that
evaluates to itself and that can be evaluated in the empty
environment. For instance,
quote(c(1, 2)) is not a literal, it is
a call. However, the result of evaluating it in
base_env() is a
literal(in this case an atomic vector).
As the data structure for function arguments, pairlists are also a
kind of language objects. However, since they are mostly an
internal data structure and can't be returned as is by the parser,
FALSE for pairlists.
is_call() for a call predicate.
q1 <- quote(1) is_expression(q1)#>  TRUEis_syntactic_literal(q1)#>  TRUEq2 <- quote(x) is_expression(q2)#>  TRUEis_symbol(q2)#>  TRUEq3 <- quote(x + 1) is_expression(q3)#>  TRUEis_call(q3)#>  TRUE# Atomic expressions are the terminating nodes of a call tree: # NULL or a scalar atomic vector: is_syntactic_literal("string")#>  TRUEis_syntactic_literal(NULL)#>  TRUEis_syntactic_literal(letters)#>  FALSE#>  FALSE#>  TRUE#>  TRUE#>  TRUE# Like any literals, they can be evaluated within the empty # environment: eval_bare(quote(1L), empty_env())#>  1# Whereas it would fail for symbolic expressions: # eval_bare(quote(c(1L, 2L)), empty_env()) # Pairlists are also language objects representing argument lists. # You will usually encounter them with extracted formals: fmls <- formals(is_expression) typeof(fmls)#>  "pairlist"# Since they are mostly an internal data structure, is_expression() # returns FALSE for pairlists, so you will have to check explicitly # for them: is_expression(fmls)#>  FALSEis_pairlist(fmls)#>  TRUE