tools that we'll show you how to develop.
Create an application that downloads podcast episodes from the Internet, and stores its
history in an SQL database.
Test your code in an intuitive and powerful way. Describe properties that ought to be true,
then let the QuickCheck library generate test cases automatically.
Take a grainy phone camera snapshot of a barcode, and turn it into an identifier that you
can use to query a library or bookseller's web site.
Write code that thrives on the web. Exchange data with servers and clients written in other
languages using JSON notation. Develop a concurrent link checker.
A little bit about you
What will you need to know before reading this book? We expect that you already know how to
program, but if you've never used a functional language, that's fine.
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.
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
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.
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
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