Functional programming in other languages

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 provided a convenient shorthand. For unary functions, ~ .x + 1 is equivalent to function(.x) .x + 1. Now we recommend using the function shorthand notation introduced in R 4.1, where \(x) 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.