Functional programming in other languages • purrr

purrr draws inspiration from many related tools:

List operations defined in the Haskell prelude
Scala’s list methods.
Functional programming libraries for javascript: underscore.js, lodash and lazy.js.
rlist, another R package to support working with lists. Similar goals but somewhat different philosophy.

However, the goal of purrr is not to try and simulate a purer functional programming language in R; we don’t want to implement a second-class version of Haskell in R. The goal is to give you similar expressiveness to an FP language, while allowing you to write code that looks and works like R:

Instead of point free (tacit) style, we use the pipe, %>%, to write code that can be read from left to right.
Instead of currying, we use ... to pass in extra arguments.
Before R 4.1, anonymous functions were verbose, so we provide two convenient shorthands. For unary functions, ~ .x + 1 is equivalent to function(.x) .x + 1.
R is weakly typed, so we need map variants that describe the output type (like map_int(), map_dbl(), etc) because we don’t know the return type of .f.
R has named arguments, so instead of providing different functions for minor variations (e.g. detect() and detectLast()) we use a named argument, .right. Type-stable functions are easy to reason about so additional arguments will never change the type of the output.