lift_xy() is a composition helper. It helps you compose
functions by lifting their domain from a kind of input to another
kind. The domain can be changed from and to a list (l), a vector
(v) and dots (d). For example,
lift_ld(fun) transforms a
function taking a list to a function taking dots.
The most important of those helpers is probably
because it allows you to transform a regular function to one that
takes a list. This is often essential for composition with purrr
functional tools. Since this is such a common function,
lift() is provided as an alias for that operation.
These functions were superseded in purrr 1.0.0 because we no longer believe "lifting" to be a mainstream operation, and we are striving to reduce purrr to its most useful core. Superseded functions will not go away, but will only receive critical bug fixes.
lift(..f, ..., .unnamed = FALSE) lift_dl(..f, ..., .unnamed = FALSE) lift_dv(..f, ..., .unnamed = FALSE) lift_vl(..f, ..., .type) lift_vd(..f, ..., .type) lift_ld(..f, ...) lift_lv(..f, ...)
A function to lift.
Default arguments for
Can be a vector mold specifying both the type and the
length of the vectors to be concatenated, such as
Here dots should be taken here in a figurative way. The lifted
functions does not need to take dots per se. The function is
simply wrapped a function in
do.call(), so instead
of taking multiple arguments, it takes a single named list or
vector which will be interpreted as its arguments. This is
particularly useful when you want to pass a row of a data frame
or a list to a function and don't want to manually pull it apart
in your function.
These factories allow a function taking a vector to take a list
or dots instead. The lifted function internally transforms its
inputs back to an atomic vector. purrr does not obey the usual R
casting rules (e.g.,
c(1, "2") produces a character
vector) and will produce an error if the types are not
compatible. Additionally, you can enforce a particular vector
type by supplying
lift_ld() turns a function that takes a list into a
function that takes dots.
lift_vd() does the same with a
function that takes an atomic vector. These factory functions are
the inverse operations of
lift_vd() internally coerces the inputs of
an atomic vector. The details of this coercion can be controlled
### Lifting from ... to list(...) or c(...) x <- list(x = c(1:100, NA, 1000), na.rm = TRUE, trim = 0.9) lift_dl(mean)(x) # You can also use the lift() alias for this common operation: lift(mean)(x) # now: exec(mean, !!!x) # Default arguments can also be specified directly in lift_dl() list(c(1:100, NA, 1000)) |> lift_dl(mean, na.rm = TRUE)() # now: mean(c(1:100, NA, 1000), na.rm = TRUE) # lift_dl() and lift_ld() are inverse of each other. # Here we transform sum() so that it takes a list fun <- sum |> lift_dl() fun(list(3, NA, 4, na.rm = TRUE)) # now: fun <- function(x) exec("sum", !!!x) exec(sum, 3, NA, 4, na.rm = TRUE) ### Lifting from c(...) to list(...) or ... # In other situations we need the vector-valued function to take a # variable number of arguments as with pmap(). This is a job for # lift_vd(): pmap_dbl(mtcars, lift_vd(mean)) # now pmap_dbl(mtcars, \(...) mean(c(...))) ### Lifting from list(...) to c(...) or ... # This kind of lifting is sometimes needed for function # composition. An example would be to use pmap() with a function # that takes a list. In the following, we use some() on each row of # a data frame to check they each contain at least one element # satisfying a condition: mtcars |> pmap_lgl(lift_ld(some, partial(`<`, 200))) # now mtcars |> pmap_lgl(\(...) any(c(...) > 200))
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