O'Reilly logo

Real World Haskell by Donald Bruce Stewart, Bryan O'Sullivan, John Goerzen

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

What to Expect from Haskell

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 favor 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.

Compared to Traditional Static Languages

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 it scales to hundreds of thousands of concurrent threads.

Compared to Modern Dynamic Languages

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 to 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 recognize that each of these approaches involves trade-offs. Very briefly put, the Haskell perspective emphasizes safety, while the dynamically typed outlook favors 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, the authors, have our own opinions about which trade-offs 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.

Haskell in Industry and Open Source

Here are just a few examples of large software systems that have been created in Haskell. Some of these are open source, while others are proprietary products:

  • ASIC and FPGA design software (Lava, products from Bluespec, Inc.)

  • Music composition software (Haskore)

  • Compilers and compiler-related tools (most notably GHC)

  • Distributed revision control (Darcs)

  • Web middleware (HAppS, products from Galois, Inc.)

The following is a sample of some of the companies using Haskell in late 2008, taken from the Haskell wiki:


An international bank. It uses Haskell in investment banking, in order to measure the counterparty risk on portfolios of financial derivatives.


A startup company. It develops multimedia content creation tools using Haskell.


A biotech company. It creates mathematical models and other complex applications in Haskell.


An ASIC and FPGA design software vendor. Its products are developed in Haskell, and the chip design languages that its products provide are influenced by Haskell.


Uses Haskell for the design and verification of hydraulic hybrid vehicle systems.

Compilation, Debugging, and Performance Analysis

For practical work, almost as important as a language itself is the ecosystem of libraries and tools around it. Haskell has a strong showing in this area.

The most widely used compiler, GHC, has been actively developed for over 15 years and provides a mature and stable set of features:

  • Compiles to efficient native code on all major modern operating systems and CPU architectures

  • Easy deployment of compiled binaries, unencumbered by licensing restrictions

  • Code coverage analysis

  • Detailed profiling of performance and memory usage

  • Thorough documentation

  • Massively scalable support for concurrent and multicore programming

  • Interactive interpreter and debugger

Bundled and Third-Party Libraries

The GHC compiler ships with a collection of useful libraries. Here are a few of the common programming needs that these libraries address:

  • File I/O and filesystem traversal and manipulation

  • Network client and server programming

  • Regular expressions and parsing

  • Concurrent programming

  • Automated testing

  • Sound and graphics

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 these open source libraries include the following:

  • Interfaces to all major open source and commercial databases

  • XML, HTML, and XQuery processing

  • Network and web client and server development

  • Desktop GUIs, including cross-platform toolkits

  • Support for Unicode and other text encodings

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required