John Goerzen on Why You Should Learn Haskell

Where does laziness help real world programs? The traditional explanations I've seen in beginning tutorials and calculate Fibonacci, which is not that interesting.

Goerzen: One of the first things that I thought was interesting was how laziness relates to IO in Haskell. In Haskell there's a function called getContents, and getContents returns a string. That string represents the entire contents of standard input or of a file or whatever, but it doesn't just read the whole thing into memory all at once. It also doesn't mmap. In other languages, if you want a string that represents an entire file, you're either going to mmap the thing or you're going to have to waste a lot of memory, or you read it in blocks or lines or whatever else you might do. If I write a little filter or parser program in Haskell, I don't have to really worry about the details of reading the input line-by-line or block-by-block because I can just use getContents and then I can string together a whole bunch of functions. I like writing Unix filters in Haskell.

That doesn't sound so extraordinary. After all, imperative languages also offer some pretty nice abstractions. The coolest aspect is not how Haskell abstracts the IO, but that lazy IO allows you to completely restructure your programs. You can (and should) separate the code that deals with the IO from code that works on the contents of a file - any kind of resource actually. In an imperative setting you tend to have a control structure that loops through the contents of a file chunk by chunk and in the middle of that you have a code block that processes each chunk of the contents. These two blocks are essentially unrelated. It had never even occurred to me until I saw the Haskell way of performing input output. It's much cleaner and also much more composible. You can simply plug in another processing function if it should become necessary at some point in the future. That is as long as the types match. And if they don't, the compiler will warn you about it preventing nasty runtime exceptions.