composition.R

#' Lift the domain of a function
#'
#' `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 `lift_dl()`
#' 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.
#'
#' @inheritParams as_vector
#' @param ..f A function to lift.
#' @param ... Default arguments for `..f`. These will be
#'   evaluated only once, when the lifting factory is called.
#' @return A function.
#' @name lift
#' @seealso [invoke()]
NULL

#' @rdname lift
#' @section from ... to `list(...)` or `c(...)`:
#'   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.
#' @param .unnamed If `TRUE`, `ld` or `lv` will not
#'   name the parameters in the lifted function signature. This
#'   prevents matching of arguments by name and match by position
#'   instead.
#' @export
#' @examples
#' ### Lifting from ... to list(...) or c(...)
#'
#' x <- list(x = c(1:100, NA, 1000), na.rm = TRUE, trim = 0.9)
#' lift_dl(mean)(x)
#'
#' # Or in a pipe:
#' mean %>% lift_dl() %>% invoke(x)
#'
#' # You can also use the lift() alias for this common operation:
#' lift(mean)(x)
#'
#' # Default arguments can also be specified directly in lift_dl()
#' list(c(1:100, NA, 1000)) %>% lift_dl(mean, 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 we transform it back to a variadic function
#' fun2 <- fun %>% lift_ld()
#' fun2(3, NA, 4, na.rm = TRUE)
#'
#' # It can sometimes be useful to make sure the lifted function's
#' # signature has no named parameters, as would be the case for a
#' # function taking only dots. The lifted function will take a list
#' # or vector but will not match its arguments to the names of the
#' # input. For instance, if you give a data frame as input to your
#' # lifted function, the names of the columns are probably not
#' # related to the function signature and should be discarded.
#' lifted_identical <- lift_dl(identical, .unnamed = TRUE)
#' mtcars[c(1, 1)] %>% lifted_identical()
#' mtcars[c(1, 2)] %>% lifted_identical()
lift <- function(..f, ..., .unnamed = FALSE) {
  force(..f)
  defaults <- list(...)
  function(.x = list(), ...) {
    if (.unnamed) {
      .x <- unname(.x)
    }
    do.call("..f", c(.x, defaults, list(...)))
  }
}

#' @rdname lift
#' @export
lift_dl <- lift

#' @rdname lift
#' @export
lift_dv <- function(..f, ..., .unnamed = FALSE) {
  force(..f)
  defaults <- list(...)

  function(.x, ...) {
    if (.unnamed) {
      .x <- unname(.x)
    }
    .x <- as.list(.x)
    do.call("..f", c(.x, defaults, list(...)))
  }
}

#' @rdname lift
#' @section from `c(...)` to `list(...)` or `...`:
#'   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 `.type`.
#' @export
#' @examples
#' #
#'
#'
#' ### 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(mtcars, lift_vd(mean))
#'
#' # lift_vd() will collect the arguments and concatenate them to a
#' # vector before passing them to ..f. You can add a check to assert
#' # the type of vector you expect:
#' lift_vd(tolower, .type = character(1))("this", "is", "ok")
lift_vl <- function(..f, ..., .type) {
  force(..f)
  defaults <- list(...)
  if (missing(.type)) .type <- NULL

  function(.x = list(), ...) {
    x <- as_vector(.x, .type)
    do.call("..f", c(list(x), defaults, list(...)))
  }
}

#' @rdname lift
#' @export
lift_vd <- function(..f, ..., .type) {
  force(..f)
  defaults <- list(...)
  if (missing(.type)) .type <- NULL

  function(...) {
    x <- as_vector(list(...), .type)
    do.call("..f", c(list(x), defaults))
  }
}

#' @rdname lift
#' @section from list(...) to c(...) or ...:
#' `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_dl()` and `lift_dv()`.
#'
#' `lift_vd()` internally coerces the inputs of `..f` to
#' an atomic vector. The details of this coercion can be controlled
#' with `.type`.
#'
#' @export
#' @examples
#' #
#'
#'
#' ### Lifting from list(...) to c(...) or ...
#'
#' # cross() normally takes a list of elements and returns their
#' # cartesian product. By lifting it you can supply the arguments as
#' # if it was a function taking dots:
#' cross_dots <- lift_ld(cross)
#' out1 <- cross(list(a = 1:2, b = c("a", "b", "c")))
#' out2 <- cross_dots(a = 1:2, b = c("a", "b", "c"))
#' identical(out1, out2)
#'
#' # 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(lift_ld(some, partial(`<`, 200)))
#'
#' # Default arguments for ..f can be specified in the call to
#' # lift_ld()
#' lift_ld(cross, .filter = `==`)(1:3, 1:3) %>% str()
#'
#'
#' # Here is another function taking a list and that we can update to
#' # take a vector:
#' glue <- function(l) {
#'   if (!is.list(l)) stop("not a list")
#'   l %>% invoke(paste, .)
#' }
#'
#' \dontrun{
#' letters %>% glue()           # fails because glue() expects a list}
#'
#' letters %>% lift_lv(glue)()  # succeeds
lift_ld <- function(..f, ...) {
  force(..f)
  defaults <- list(...)
  function(...) {
    do.call("..f", c(list(list(...)), defaults))
  }
}

#' @rdname lift
#' @export
lift_lv <- function(..f, ...) {
  force(..f)
  defaults <- list(...)
  function(.x, ...) {
    do.call("..f", c(list(as.list(.x)), defaults, list(...)))
  }
}