Table of Contents
Haskell is a deep language, and we think that learning it is a hugely rewarding experience. We will focus on three elements as we explain why. The first is novelty: we invite you to think about programming from a different and valuable perspective. The second is power: we'll show you how to create software that is short, fast, and safe. Lastly, we offer you a lot of fun: the pleasure of applying beautiful programming techniques to solve real problems.
Haskell is most likely quite different from any language you've ever used before. Compared to the usual set of concepts in a programmer's mental toolbox, functional programming offers us a profoundly different way to think about software.
In Haskell, we de-emphasise code that modifies data. Instead, we focus on functions that take immutable values as input and produce new values as output. Given the same inputs, these functions always return the same results. This is a core idea behind functional programming.
Along with not modifying data, our Haskell functions usually don't talk to the external world; we call these functions pure. We make a strong distinction between pure code and the parts of our programs that read or write files, communicate over network connections, or make robot arms move. This makes it easier to organize, reason about, and test our programs.
Even the way in which we evaluate expressions is different in Haskell. We defer every computation until its result is actually needed: Haskell is a lazy language. Laziness is not merely a matter of moving work around: it profoundly affects how we write programs.
Throughout this book, we will show you how Haskell's alternatives to the features of traditional languages are powerful, flexible, and lead to reliable code. Haskell is positively crammed full of cutting edge ideas about how to create great software.
Since pure code has no dealings with the outside world, and the data it works with is never modified, the kinds of nasty surprise in which one piece of code invisibly corrupts data used by another are very rare. Whatever context we use a pure function in, it will behave consistently.
Pure code is easier to test than code that deals with the outside world. When a function only responds to its visible inputs, we can easily state properties of its behavior that should always be true. We can automatically test that those properties hold for a huge body of random inputs, and when our tests pass, we move on. We still use traditional techniques to test code that must interact with files, networks, or exotic hardware. Since there is much less of this impure code than we would find in a traditional language, we gain much more assurance that our software is solid.
Lazy evaluation has some spooky effects. Let's say we want to find the k least-valued elements of an unsorted list. In a traditional language, the obvious approach would be to sort the list and take the first k elements, but this is expensive. For efficiency, we would instead write a special function that takes these values in one pass, and it would have to perform some moderately complex book-keeping. In Haskell, the sort-then-take approach actually performs well: laziness ensures that the list will only be sorted enough to find the k minimal elements.
-- file: ch00/KMinima.hs -- lines beginning with "--" are comments. minima k xs = take k (sort xs)
As the above example shows, an important aspect of Haskell's power lies in the compactness of the code we write. Compared to working in popular traditional languages, when we develop in Haskell we often write much less code, in substantially less time, and with fewer bugs.
Since effective programming in Haskell differs greatly from other languages, you should expect that mastering both the language itself and functional programming techniques will require plenty of thought and practice.
Harking back to our own days of getting started with Haskell, the good news is that the fun begins early: it's simply an entertaining challenge to dig into a new language, in which so many commonplace ideas are different or missing, and to figure out how to write simple programs.
For us, the initial pleasure lasted as our experience grew and our understanding deepened. In other languages, it's difficult to see any connection between science and the nuts-and-bolts of programming. In Haskell, we have imported some ideas from abstract mathematics and put them to work. Even better, we find that not only are these ideas easy to pick up, they have a practical payoff in helping us to write more compact, reusable code.
That being said, Haskell is a rigorous language: it will make you perform more of your thinking up front. It can take a little while to adjust to debugging much of your code before you ever run it, in response to the compiler telling you that something about your program does not make sense. Even with years of experience, we remain astonished and pleased by how often our Haskell programs simply work on the first try, once we fix those compilation errors.
We started this project because a growing number of people are using Haskell to solve everyday problems. Because Haskell has its roots in academia, few of the Haskell books that currently exist focus on the problems and techniques of everyday programming that we're interested in.
With this book, we want to show you how to use functional programming and Haskell to solve realistic problems. This is a hands-on book: every chapter contains dozens of code samples, and many contain complete applications. Here are a few examples of the libraries, techniques and tools that we'll show you how to develop.
No matter what your level of experience is, we have tried to anticipate your needs: we go out of our way to explain new and potentially tricky ideas in depth, usually with examples and images to drive our points home.
As a new Haskell programmer, you'll inevitably start out writing quite a bit of code by hand for which you could have used a library function or programming technique, had you just known of its existence. We've packed this book with information to help you to come up to speed as quickly as possible.
Of course, there will always be a few bumps along the road. If you start out anticipating an occasional surprise or difficulty along with the fun stuff, you will have the best experience. Any rough patches you might hit won't last long.
As you become a more seasoned Haskell programmer, the way that you write code will change. Indeed, over the course of this book, the way that we present code will evolve, as we move from the basics of the language to increasingly powerful and productive features and techniques.
Haskell is a general purpose programming language. It was designed without any application niche in mind. Although it takes a strong stand on how programs should be written, it does not favour one problem domain over others.
While at its core, the language encourages a pure, lazy style of functional programming, this is the default, not the only option. Haskell also supports the more traditional models of procedural code and strict evaluation. Additionally, although the focus of the language is squarely on writing statically typed programs, it is possible (though rarely seen) to write Haskell code in a dynamically typed manner.
Languages that use simple static type systems have been the mainstay of the programming world for decades. Haskell is statically typed, but its notion of what types are for, and what we can do with them, is much more flexible and powerful than traditional languages. Types make a major contribution to the brevity, clarity, and efficiency of Haskell programs.
Although powerful, Haskell's type system is often also unobtrusive. If we omit explicit type information, a Haskell compiler will automatically infer the type of an expression or function. Compared to traditional static languages, to which we must spoon-feed large amounts of type information, the combination of power and inference in Haskell's type system significantly reduces the clutter and redundancy of our code.
Several of Haskell's other features combine to further increase the amount of work we can fit into a screenful of text. This brings improvements in development time and agility: we can create reliable code quickly, and easily refactor it in response to changing requirements.
Sometimes, Haskell programs may run more slowly than similar programs written in C or C++. For most of the code we write, Haskell's large advantages in productivity and reliability outweigh any small performance disadvantage.
Multicore processors are now ubiquitous, but they remain notoriously difficult to program using traditional techniques. Haskell provides unique technologies to make multicore programming more tractable. It supports parallel programming, software transactional memory for reliable concurrency, and scales to hundreds of thousands of concurrent threads.
Over the past decade, dynamically typed, interpreted languages have become increasingly popular. They offer substantial benefits in developer productivity. Although this often comes at the cost of a huge performance hit, for many programming tasks productivity trumps performance, or performance isn't a significant factor in any case.
Brevity is one area in which Haskell and dynamically typed languages perform similarly: in each case, we write much less code to solve a problem than in a traditional language. Programs are often around the same size in dynamically typed languages and Haskell.
When we consider runtime performance, Haskell almost always has a huge advantage. Code compiled by the Glasgow Haskell Compiler (GHC) is typically between 20 and 60 times faster than code run through a dynamic language's interpreter. GHC also provides an interpreter, so you can run scripts without compiling them.
Another big difference between dynamically typed languages and Haskell lies in their philosophies around types. A major reason for the popularity of dynamically typed languages is that only rarely do we need to explicitly mention types. Through automatic type inference, Haskell offers the same advantage.
Beyond this surface similarity, the differences run deep. In a dynamically typed language, we can create constructs that are difficult to express in a statically typed language. However, the same is true in reverse: with a type system as powerful as Haskell's, we can structure a program in a way that would be unmanageable or infeasible in a dynamically typed language.
It's important to recognise that each of these approaches involves tradeoffs. Very briefly put, the Haskell perspective emphasises safety, while the dynamically typed outlook favours flexibility. If someone had already discovered one way of thinking about types that was always best, we imagine that everyone would know about it by now.
Of course, we have our own opinions about which tradeoffs are more beneficial. Two of us have years of experience programming in dynamically typed languages. We love working with them; we still use them every day; but usually, we prefer Haskell.
is a sample of some of the companies using Haskell in late 2008, taken from the Haskell wiki.
The Hackage package database is the Haskell community's collection of open source libraries and applications. Most libraries published on Hackage are licensed under liberal terms that permit both commercial and open source use. Some of the areas covered by open source libraries include the following.
A few decades before modern computers were invented, the mathematician Alonzo Church developed a language called the lambda calculus. He intended it as a tool for investigating the foundations of mathematics. The first person to realize the practical connection between programming and the lambda calculus was John McCarthy, who created Lisp in 1958.
During the 1960s, computer scientists began to recognise and study the importance of the lambda calculus. Peter Landin and Christopher Strachey developed ideas about the foundations of programming languages: how to reason about what they do (operational semantics) and how to understand what they mean (denotational semantics).
In the early 1970s, Robin Milner created a more rigorous functional programming language named ML. While ML was developed to help with automated proofs of mathematical theorems, it gained a following for more general computing tasks.
The 1970s saw the emergence of lazy evaluation as a novel strategy. David Turner developed SASL and KRC, while Rod Burstall and John Darlington developed NPL and Hope. NPL, KRC and ML influenced the development of several more languages in the 1980s, including Lazy ML, Clean, and Miranda.
By the late 1980s, the efforts of researchers working on lazy functional languages were scattered across more than a dozen languages. Concerned by this diffusion of effort, a number of researchers decided to form a committee to design a common language. After three years of work, the committee published the Haskell 1.0 specification in 1990. It named the language after Haskell Curry, an influential logician.
Many people are rightfully suspicious of “design by committee”, but the work of the Haskell committee is a beautiful example of the best work a committee can do. They produced an elegant, considered language design, and succeeded in unifying the fractured efforts of their research community. Of the thicket of lazy functional languages that existed in 1990, only Haskell is still actively used.
Since its publication in 1990, the Haskell language standard has seen five revisions, most recently in 1998. A number of Haskell implementations have been written, and several are still actively developed.
During the 1990s, Haskell served two main purposes. On one side, it gave language researchers a stable language in which to experiment with making lazy functional programs run efficiently. Other researchers explored how to construct programs using lazy functional techniques. Still others used it as a teaching language.
While these basic explorations of the 1990s proceeded, Haskell remained firmly an academic affair. The informal slogan of those inside the community was to “avoid success at all costs”. Few outsiders had heard of the language at all. Indeed, functional programming as a field was quite obscure.
During this time, the mainstream programming world experimented with relatively small tweaks: from programming in C, to C++, to Java. Meanwhile, on the fringes, programmers were beginning to tinker with new, more dynamic languages. Guido van Rossum designed Python; Larry Wall created Perl; and Yukihiro Matsumoto developed Ruby.
As these newer languages began to seep into wider use, they spread some crucial ideas. The first was that programmers are not merely capable of working in expressive languages; in fact, they flourish. The second was in part a byproduct of the rapid growth in raw computing power of that era: it's often smart to sacrifice some execution performance in exchange for a big increase in programmer productivity. Finally, several of these languages borrowed from functional programming.
As you work with Haskell, you're sure to have questions and want more information about things. Here are some Internet resources where you can look up information and interact with other Haskell programmers.
The Haskell Hierarchical Libraries reference provides the documentation for the standard library that comes with your compiler. This is one of the most valuable online assets for Haskell programmers.
For questions about language syntax and features, the Haskell 98 Report describes the Haskell 98 language standard.
Various extensions to the language have become commonplace since the Haskell 98 Report was released. The GHC Users's Guide contains detailed documentation on the extensions supported by GHC, as well as some GHC-specific features.
The Haskell community maintains a central repository of open source Haskell libraries and applications. It's called Hackage, and it lets you search for software to download, or browse its collection by category.
The Haskell Wiki contains a section dedicated to information about particular Haskell libraries.
The first stop on your search for community resources should be the Haskell web site. This page contains the most current links to various communities and information, as well as a huge and actively maintained wiki.
Haskellers use a number of mailing
lists for topical discussions. Of these, the
most generally interesting is named
haskell-cafe. It has a relaxed,
friendly atmosphere, where professionals and academics rub
shoulders with casual hackers and beginners.
For real-time chat, the Haskell
IRC channel, named
large and lively. Like
atmosphere stays friendly and helpful in spite of the huge
number of concurrent users.
There are many local user groups, meetups, academic workshops, and the like; here is a list of the known user groups and workshops.
The Haskell Weekly News is a very-nearly-weekly summary of activities in the Haskell community. You can find pointers to interesting mailing list discussions, new software releases, and the like.
The Haskell Communities and Activities Report collects information about people that use Haskell, and what they are doing with it. It has been running for years, so it provides a good way to peer into Haskell's past.
This book would not exist without the Haskell community: an anarchic, hopeful cabal of artists, theoreticians and engineers, who for twenty years have worked to create a better, bug-free programming world. The people of the Haskell community are unique in their combination of friendliness and intellectual depth.
Simon Peyton Jones took a chance on a college student who emailed him out of the blue in early 1994. Interning for him over that summer remains a highlight of my professional life. With his generosity, boundless energy, and drive to collaborate, he inspires the whole Haskell community.
I am so glad to be able to work with Bryan and Don on this project. The depth of their Haskell knowledge and experience is amazing. I enjoyed finally being able to have the three of us sit down in the same room -- over a year after we started writing.
My 2-year-old Jacob, who decided that it would be fun to use a keyboard too, and is always eager to have me take a break from the computer and help him make some fun typing noises on a 50-year-old Underwood typewriter.
My PhD supervisor, Manuel Chakravarty, and the PLS research group, who provided encouragement, vision and energy, and showed me that a rigorous, foundational approach to programming can make the impossible happen.
We developed this book in the open, posting drafts of chapters to our web site as we completed them. Readers then submitted feedback using a web application that we developed. By the time we finished writing the book, about 800 people had submitted over 7,500 comments, an astounding figure.
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