rlang (version 0.2.2)

is_expression: Is an object an expression?

Description

is_expression() tests for expressions, the set of objects that can be obtained from parsing R code. An expression can be one of two things: either a symbolic object (for which is_symbolic() returns TRUE), or a syntactic literal (testable with is_syntactic_literal()). Technically, calls can contain any R object, not necessarily symbolic objects or syntactic literals. However, this only happens in artificial situations. Expressions as we define them only contain numbers, strings, NULL, symbols, and calls: this is the complete set of R objects that can be created when R parses source code (e.g. from using parse_expr()).

Note that we are using the term expression in its colloquial sense and not to refer to expression() vectors, a data type that wraps expressions in a vector and which isn't used much in modern R code.

Usage

is_expression(x)
is_syntactic_literal(x)
is_symbolic(x)

Arguments

x

An object to test.

Details

is_symbolic() returns TRUE for symbols and calls (objects with type 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 evaluation.

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).

Pairlists are also a kind of language objects. However, since they are mostly an internal data structure, is_expression() returns FALSE for pairlists. You can use is_pairlist() to explicitly check for them. Pairlists are the data structure for function arguments. They usually do not arise from R code because subsetting a call is a type-preserving operation. However, you can obtain the pairlist of arguments by taking the CDR of the call object from C code. The rlang function node_cdr() will do it from R. Another way in which pairlist of arguments arise is by extracting the argument list of a closure with base::formals() or fn_fmls().

See Also

is_call() for a call predicate.

Examples

# NOT RUN {
q1 <- quote(1)
is_expression(q1)
is_syntactic_literal(q1)

q2 <- quote(x)
is_expression(q2)
is_symbol(q2)

q3 <- quote(x + 1)
is_expression(q3)
is_call(q3)


# Atomic expressions are the terminating nodes of a call tree:
# NULL or a scalar atomic vector:
is_syntactic_literal("string")
is_syntactic_literal(NULL)

is_syntactic_literal(letters)
is_syntactic_literal(quote(call()))

# Parsable literals have the property of being self-quoting:
identical("foo", quote("foo"))
identical(1L, quote(1L))
identical(NULL, quote(NULL))

# Like any literals, they can be evaluated within the empty
# environment:
eval_bare(quote(1L), empty_env())

# 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)

# 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)
is_pairlist(fmls)
# }