rlang (version 0.1)

dots_definitions: Tidy quotation of multiple expressions and dots

Description

quos() quotes its arguments and returns them as a list of quosures (see quo()). It is especially useful to capture arguments forwarded through ....

Usage

dots_definitions(..., .named = FALSE)
quos(..., .named = FALSE, .ignore_empty = c("trailing", "none", "all"))
is_quosures(x)

Arguments

...

Expressions to capture unevaluated.

.named

Whether to ensure all dots are named. Unnamed elements are processed with expr_text() to figure out a default name. If an integer, it is passed to the width argument of expr_text(), if TRUE, the default width is used. See exprs_auto_name().

.ignore_empty

Whether to ignore empty arguments. Can be one of "trailing", "none", "all". If "trailing", only the last argument is ignored if it is empty.

x

An object to test.

Details

Both quos and dots_definitions() have specific support for definition expressions of the type var := expr, with some differences:

quos()

When := definitions are supplied to quos(), they are treated as a synonym of argument assignment =. On the other hand, they allow unquoting operators on the left-hand side, which makes it easy to assign names programmatically.

dots_definitions()

This dots capturing function returns definitions as is. Unquote operators are processed on capture, in both the LHS and the RHS. Unlike quos(), it allows named definitions.

Examples

# NOT RUN {
# quos() is like the singular version but allows quoting
# several arguments:
quos(foo(), bar(baz), letters[1:2], !! letters[1:2])

# It is most useful when used with dots. This allows quoting
# expressions across different levels of function calls:
fn <- function(...) quos(...)
fn(foo(bar), baz)

# Note that quos() does not check for duplicate named
# arguments:
fn <- function(...) quos(x = x, ...)
fn(x = a + b)


# Dots can be spliced in:
args <- list(x = 1:3, y = ~var)
quos(!!! args, z = 10L)

# Raw expressions are turned to formulas:
args <- alist(x = foo, y = bar)
quos(!!! args)


# Definitions are treated similarly to named arguments:
quos(x := expr, y = expr)

# However, the LHS of definitions can be unquoted. The return value
# must be a symbol or a string:
var <- "foo"
quos(!!var := expr)

# If you need the full LHS expression, use dots_definitions():
dots <- dots_definitions(var = foo(baz) := bar(baz))
dots$defs
# }