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 overused 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 underplayed 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 much more besides.