5. Reflections on Haskell

Haskell is, first and foremost, a functional language. Nevertheless, I think that it is also the world’s most beautiful imperative language. Considered as an imperative language, Haskell’s unusual features are that

  • Actions (which have effects) are rigorously distinguished from pure values by the type system.
  • Actions are first-class values. They can be passed to functions, returned as results, formed into lists, and so on, all without causing any side effects.

Using actions as first-class values, the programmer can define application-specific control structures, rather than make do with the ones provided by the language designer. For example, nTimes is a simple for loop, and choose implements a sort of guarded command. We also saw other applications of actions as values. In the main program we used Haskell’s rich expression language (in this case list comprehensions) to generate a list of actions, which we then performed in order, using sequence_. Earlier, when defining helper1, we improved modularity by abstracting out an action from a chunk of code. To illustrate these points I have perhaps over-used Haskell’s abstraction power in the Santa code, which is a very small program. For large programs, though, it is hard to overstate the importance of actions as values.

On the other hand, I have under-played other aspects of Haskell — higher order functions, lazy evaluation, data types, polymorphism, type classes, and so on — because of the focus on concurrency. Not many Haskell programs are as imperative as this one! You can find a great deal of information about Haskell at http://haskell.org, including books, tutorials, Haskell compilers and interpreters, Haskell libraries, mailing lists and so on. Plus, of course, all the tutorials here at the School of Haskell!

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