CHAPTER 1 - INTRODUCTION

PROGRAMMING IN HASKELL

What is a functional language?

Opinions differ, and it is difficult to give a precise definition, but generally speaking:

1)Functional programming is style of programming in which the basic method of computation is the application of functions to arguments.

2)A functional language is one that supports and encourages the functional style.

Example:

Summing the integers 1 to 10 in Java

int total = 0; for (int i = 1; i <= 10; i++) total = total + i;

The computation method is variable assignment.

Example:

Summing the integers 1 to 10 in Haskell:

sum [1..10]

55

The computation method is function application.

Historical Background

1930s: Alonzo Church develops the lambda calculus, a simple but powerful theory of functions.

1950s: John McCarthy develops Lisp, the first functional language, with some influences from the lambda calculus, but retaining variable assignments.

1960s: Peter Landin develops ISWIM, the first pure functional language, based strongly on the lambda calculus, with no assignments.

1970s: John Backus develops FP, a functional language that emphasizes higher-order functions and reasoning about programs.

1970s: Robin Milner and others develop ML, the first modern functional language, which introduced type inference and polymorphic types.

1970s - 1980s: David Turner develops a number of lazy functional languages, culminating in the Miranda system.

1987: An international committee starts the development of Haskell, a standard lazy functional language.

1990s: Phil Wadler and others develop type classes and monads, two of the main innovations of Haskell.

2003: The committee publishes the Haskell Report, defining a stable version of the language; an updated version was published in 2010.

2010-date: Standard distribution, library support, new language features, development tools, use in industry, influence on other languages, etc.

A Taste of Haskell

f []     = []
f (x:xs) = f ys ++ [x] ++ f zs
           where
              ys = [a | a <- xs, a <= x]
              zs = [b | b <- xs, b > x]

The above code is an example of quick sort in HASKELL.