rlang (version 0.2.2)

expr_interp: Process unquote operators in a captured expression

Description

While all capturing functions in the tidy evaluation framework perform unquote on capture (most notably quo()), expr_interp() manually processes unquoting operators in expressions that are already captured. expr_interp() should be called in all user-facing functions expecting a formula as argument to provide the same quasiquotation functionality as NSE functions.

Usage

expr_interp(x, env = NULL)

Arguments

x

A function, raw expression, or formula to interpolate.

env

The environment in which unquoted expressions should be evaluated. By default, the formula or closure environment if a formula or a function, or the current environment otherwise.

Examples

# NOT RUN {
# All tidy NSE functions like quo() unquote on capture:
quo(list(!!(1 + 2)))

# expr_interp() is meant to provide the same functionality when you
# have a formula or expression that might contain unquoting
# operators:
f <- ~list(!!(1 + 2))
expr_interp(f)

# Note that only the outer formula is unquoted (which is a reason
# to use expr_interp() as early as possible in all user-facing
# functions):
f <- ~list(~!!(1 + 2), !!(1 + 2))
expr_interp(f)


# Another purpose for expr_interp() is to interpolate a closure's
# body. This is useful to inline a function within another. The
# important limitation is that all formal arguments of the inlined
# function should be defined in the receiving function:
other_fn <- function(x) toupper(x)

fn <- expr_interp(function(x) {
  x <- paste0(x, "_suffix")
  !!! body(other_fn)
})
fn
fn("foo")
# }