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,
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!