Introduction

Most languages let functions do anything: change variables, print, read files, crash. Haskell, released in 1990, forbids most of that — and programmers who learn it often say it changed how they write every other language. It is a purely functional language: functions have no side effects, data never changes, and the type system catches whole classes of bugs before the program runs. It is less a tool for every job than a way of thinking that leaks, usefully, into everything else you write.

Purity, types, and laziness

Haskell's model rests on a few distinctive ideas:

Purity — a function, given the same inputs, always returns the same output and changes nothing.…