# 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 librarys 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.

- Anonymous functions are verbose in R, so we provide two convenient shorthands. For unary functions, `~ .x + 1` is equivalent to `function(.x) .x + 1`. For chains of transformations functions, `. %>% f() %>% g()` is equivalent to `function(.) . %>% f() %>% g()` (this shortcut is provided by magrittr).

- R is weakly typed, so we need `map` variants that describe the output type (like `map_int()`, `map_dbl()`, etc) because don’t 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.