Partial function application allows you to modify a function by pre-filling some of the arguments. It is particularly useful in conjunction with functionals and other function operators.

Note that an argument can only be partialised once.

partial(.f, ..., .env = NULL, .lazy = NULL, .first = NULL)

Arguments

.f

a function. For the output source to read well, this should be a named function.

...

named arguments to .f that should be partially applied.

Pass an empty ... = argument to specify the position of future arguments relative to partialised ones. See rlang::call_modify() to learn more about this syntax.

These dots support quasiquotation and quosures. If you unquote a value, it is evaluated only once at function creation time. Otherwise, it is evaluated each time the function is called.

.env

Soft-deprecated as of purrr 0.3.0. The environments are now captured via quosures.

.lazy

Soft-deprecated as of purrr 0.3.0. Please unquote the arguments that should be evaluated once at function creation time.

.first

Soft-deprecated as of purrr 0.3.0. Please pass an empty argument ... = to specify the position of future arguments.

Examples

# Partial is designed to replace the use of anonymous functions for # filling in function arguments. Instead of: compact1 <- function(x) discard(x, is.null) # we can write: compact2 <- partial(discard, .p = is.null) # partial() works fine with functions that do non-standard # evaluation my_long_variable <- 1:10 plot2 <- partial(plot, my_long_variable) plot2()
#> NULL
plot2(runif(10), type = "l")
#> NULL
# Note that you currently can't partialise arguments multiple times: my_mean <- partial(mean, na.rm = TRUE) my_mean <- partial(my_mean, na.rm = FALSE) try(my_mean(1:10))
#> Error in mean.default(na.rm = TRUE, ...) : #> formal argument "na.rm" matched by multiple actual arguments
# The evaluation of arguments normally occurs "lazily". Concretely, # this means that arguments are repeatedly evaluated across invokations: f <- partial(runif, n = rpois(1, 5)) f
#> <partialised> #> function (...) #> runif(n = rpois(1, 5), ...)
f()
#> [1] 0.1538388 0.9316670 0.6664174 0.5214033 0.9335608
f()
#> [1] 0.39328750 0.67795588 0.80827452 0.52697525 0.11193650 0.95423720 0.02919632
# You can unquote an argument to fix it to a particular value. # Unquoted arguments are evaluated only once when the function is created: f <- partial(runif, n = !!rpois(1, 5)) f
#> <partialised> #> function (...) #> runif(n = 3L, ...)
f()
#> [1] 0.2054296 0.8999639 0.7739089
f()
#> [1] 0.9676727 0.7804088 0.9705454
# By default, partialised arguments are passed before new ones: my_list <- partial(list, 1, 2) my_list("foo")
#> [[1]] #> [1] 1 #> #> [[2]] #> [1] 2 #> #> [[3]] #> [1] "foo" #>
# Control the position of these arguments by passing an empty # `... = ` argument: my_list <- partial(list, 1, ... = , 2) my_list("foo")
#> [[1]] #> [1] 1 #> #> [[2]] #> [1] "foo" #> #> [[3]] #> [1] 2 #>