Merge pull request #145 from michaelquinn32/master

Implemented accumulate for recursive folding

DESCRIPTION

@@ -23,4 +23,4 @@ LinkingTo: Rcpp,
     BH
 URL: https://github.com/hadley/purrr
 BugReports: https://github.com/hadley/purrr/issues
-RoxygenNote: 5.0.0
+RoxygenNote: 5.0.1

NAMESPACE

@@ -3,6 +3,8 @@
 export("%>%")
 export("%@%")
 export("%||%")
+export(accumulate)
+export(accumulate_right)
 export(array_branch)
 export(array_tree)
 export(as_function)

NEWS.md

@@ -52,3 +52,7 @@
 
 * `zip2()`, `zip3()`, and `zip_n()` have been replaced by `transpose()`.
   It does the same thing but the name is better (#128).
+
+* `accumulate()` has been added to handle recursive folding. It is shortand
+  for `Reduce(f, .x, accumulate = TRUE)` and follows a similar syntax to 
+  `reduce()` (#145). A right-hand version `accumulate_right()` was also added.

R/reduce.R

@@ -29,20 +29,66 @@
 #' list() %>% reduce(`+`)
 #' list() %>% reduce(`+`, .init = 0)
 reduce <- function(.x, .f, ..., .init) {
-  force(.f)
-  f <- function(x, y) .f(x, y, ...)
-
+  f <- as_function(.f)
   Reduce(f, .x, init = .init)
 }
 
 #' @export
 #' @rdname reduce
 reduce_right <- function(.x, .f, ..., .init) {
-  force(.f)
+  .f <- as_function(.f)
+
   # Note the order of arguments is switched
   f <- function(x, y) {
     .f(y, x, ...)
   }
 
   Reduce(f, .x, init = .init, right = TRUE)
 }
+
+#' Accumulate recursive folds across a list
+#'
+#' \code{accumulate} applies a function recursively over a list from the left, while
+#' \code{accumulate_right} applies the function from the right. Unlike \code{reduce}
+#' both functions keep the intermediate results.
+#'
+#' @inheritParams reduce
+#' @export
+#'
+#' @examples
+#' 1:3 %>% accumulate(`+`)
+#' 1:10 %>% accumulate_right(`*`)
+#'
+#' # From Haskell's scanl documentation
+#' 1:10 %>% accumulate(max, .init = 5)
+#'
+#' # Simulating stochastic processes with drift
+#' \dontrun{
+#' library(dplyr)
+#' library(ggplot2)
+#'
+#' rerun(5, rnorm(100)) %>%
+#'   set_names(paste0("sim", 1:5)) %>%
+#'   map(~ accumulate(., ~ .05 + .x + .y)) %>%
+#'   map_df(~ data_frame(value = .x, step = 1:100), .id = "simulation") %>%
+#'   ggplot(aes(x = step, y = value)) +
+#'     geom_line(aes(color = simulation)) +
+#'     ggtitle("Simulations of a random walk with drift")
+#' }
+accumulate <- function(.x, .f, ..., .init) {
+  f <- as_function(.f)
+  Reduce(f, .x, init = .init, accumulate = TRUE)
+}
+
+#' @export
+#' @rdname accumulate
+accumulate_right <- function(.x, .f, ..., .init) {
+  .f <- as_function(.f)
+
+  # Note the order of arguments is switched
+  f <- function(x, y) {
+    .f(y, x, ...)
+  }
+
+  Reduce(f, .x, init = .init, right = TRUE, accumulate = TRUE)
+}