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.

expr_interp(x, env = NULL)



A function, raw expression, or formula to interpolate.


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.


# All tidy NSE functions like quo() unquote on capture: quo(list(!! 1 + 2))
#> <quosure: local> #> ~list(3)
# 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)
#> <quosure: local> #> ~list(3)
# 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)
#> <quosure: local> #> ~list(~3, 3)
# 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
#> function (x) #> { #> x <- paste0(x, "_suffix") #> toupper(x) #> } #> <environment: 0x7f979f294030>
#> [1] "FOO_SUFFIX"