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Haskell 7 : Modules

A Haskell module is a collection of related functions, types and typeclasses. A Haskell program is a collection of modules where the main module loads up the other modules and then uses the functions defined in them to do something. Having code split up into several modules has quite a lot of advantages. If a module is generic enough, the functions it exports can be used in a multitude of different programs. If your own code is separated into self-contained modules which don't rely on each other too much (we also say they are loosely coupled), you can reuse them later on. It makes the whole deal of writing code more manageable by having it split into several parts, each of which has some sort of purpose.

The Haskell standard library is split into modules, each of them contains functions and types that are somehow related and serve some common purpose. There's a module for manipulating lists, a module for concurrent programming, a module for dealing with complex numbers, etc. All the functions, types and typeclasses that we've dealt with so far were part of the Prelude module, which is imported by default. In this chapter, we're going to examine a few useful modules and the functions that they have. But first, we're going to see how to import modules.

The syntax for importing modules in a Haskell script is import ＜module name＞. This must be done before defining any functions, so imports are usually done at the top of the file. One script can, of course, import several modules. Just put each import statement into a separate line. Let's import the Data.List module, which has a bunch of useful functions for working with lists and use a function that it exports to create a function that tells us how many unique elements a list has.

import Data.List

numUniques :: (Eq a) =＞ [a] -＞ Int
numUniques = length . nub

When you do import Data.List, all the functions that Data.List exports become available in the global namespace, meaning that you can call them from wherever in the script. nub is a function defined in Data.List that takes a list and weeds out duplicate elements. Composing length and nub by doing length . nub produces a function that's the equivalent of \xs -＞ length (nub xs).