OSullivan - Real World Haskell

Throughout this book, we will show you how Haskells alternatives to the features of traditional languages are powerful and flexible and lead to reliable code. Haskell is positively crammed full of cutting-edge ideas about how to create great software.

Since pure code has no dealings with the outside world, and the data it works with is never modified, the kind of nasty surprise in which one piece of code invisibly corrupts data used by another is very rare. Whatever context we use a pure function in, the function will behave consistently.

Pure code is easier to test than code that deals with the outside world. When a function responds only to its visible inputs, we can easily state properties of its behavior that should always be true. We can automatically test that those properties hold for a huge body of random inputs, and when our tests pass, we move on. We still use traditional techniques to test code that must interact with files, networks, or exotic hardware. Since there is much less of this impure code than we would find in a traditional language, we gain much more assurance that our software is solid.

Lazy evaluation has some spooky effects. Lets say we want to find the k least-valued elements of an unsorted list. In a traditional language, the obvious approach would be to sort the list and take the first k elements, but this is expensive. For efficiency, we would instead write a special function that takes these values in one pass, and that would have to perform some moderately complex bookkeeping. In Haskell, the sort-then-take approach actually performs well: laziness ensures that the list will only be sorted enough to find the k minimal elements.

Better yet, our Haskell code that operates so efficiently is tiny and uses standard library functions:

It can take a while to develop an intuitive feel for when lazy evaluation is important, but when we exploit it, the resulting code is often clean, brief, and efficient.

As the preceding example shows, an important aspect of Haskells power lies in the compactness of the code we write. Compared to working in popular traditional languages, when we develop in Haskell we often write much less code, in substantially less time and with fewer bugs.