# Parallelize 'purrr' functions

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The futurize package allows you to easily turn sequential code into parallel code by piping the sequential code to the `futurize()` function. Easy!

# TL;DR

    library(futurize)
    plan(multisession)
    library(purrr)

    slow_fcn <- function(x) {
      Sys.sleep(0.1)  # emulate work
      x^2
    }

    xs <- 1:1000
    ys <- xs |> map(slow_fcn) |> futurize()

# Introduction

This vignette demonstrates how to use this approach to parallelize purrr functions such as `map()`, `map_dbl()`, and `walk()`.

The purrr `map()` function is commonly used to apply a function to the elements of a vector or a list. For example,

    library(purrr)
    xs <- 1:1000
    ys <- map(xs, slow_fcn)

or equivalently using pipe syntax

    library(purrr)
    xs <- 1:1000
    ys <- xs |> map(slow_fcn)

Here `map()` evaluates sequentially, but we can easily make it evaluate in parallel, by using:

    library(futurize)
    library(purrr)
    xs <- 1:1000
    ys <- xs |> map(slow_fcn) |> futurize()

This will distribute the calculations across the available parallel workers, given that we have set parallel workers, e.g.

    plan(multisession)

The built-in `multisession` backend parallelizes on your local computer and it works on all operating systems. There are [other parallel backends] to choose from, including alternatives to parallelize locally as well as distributed across remote machines, e.g.

    plan(future.mirai::mirai_multisession)

and

    plan(future.batchtools::batchtools_slurm)

Another example is:

    library(purrr)
    library(futurize)
    plan(future.mirai::mirai_multisession)

    ys <- 1:10 |>
            map(rnorm, n = 10) |> futurize(seed = TRUE) |>
            map_dbl(mean) |> futurize()

# Supported Functions

The `futurize()` function supports parallelization of the following purrr functions:

- `map()`, `map_chr()`, `map_dbl()`, `map_int()`, `map_lgl()`, `map_raw()`, `map_dfr()`, `map_dfc()`, `walk()`
- `map2()`, `map2_chr()`, `map2_dbl()`, `map2_int()`, `map2_lgl()`, `map2_raw()`, `map2_dfr()`, `map2_dfc()`, `walk2()`
- `pmap()`, `pmap_chr()`, `pmap_dbl()`, `pmap_int()`, `pmap_lgl()`, `pmap_raw()`, `pmap_dfr()`, `pmap_dfc()`, `pwalk()` `imap()`, `imap_chr()`, `imap_dbl()`, `imap_int()`, `imap_lgl()`, `imap_raw()`, `imap_dfr()`, `imap_dfc()`, `iwalk()`
- `modify()`, `modify_if()`, `modify_at()`
- `map_if()`, `map_at()`
- `invoke_map()`, `invoke_map_chr()`, `invoke_map_dbl()`, `invoke_map_int()`, `invoke_map_lgl()`, `invoke_map_raw()`, `invoke_map_dfr()`, `invoke_map_dfc()`