I quickly jot down the first draft of this article and immediately discarded it because I realized I’m preaching to the choir: people that use Haskell would already know what I was about to tell them and people that don’t use it would find lines of code to be a terrible benchmark to draw any valuable conclusions from (and they’d be right). You are making a bigger impact than you even know. Where once a dab of a damp tissue would have done, critics say wet wipes are being used when they are not needed, such as wiping hands after a sandwich or as a substitute for toilet paper. If you open the interpreter’s source code you’ll notice that functions very rarely have type declarations. Notice how the Java version is already much longer than the Haskell one despite the fact that I took obvious shortcuts and avoided writing constructors and accessor methods. Notice that the declaration is separate from the rest of the code – it almost gives you a hint that it isn’t always necessary. The four lines after the type declaration specify four cases of pattern matching and what the value of the eval function should be in those cases.
Sometimes it will get confused and you’ll have to add a type declaration but that’s an exception rather than the rule. Most of the time you don’t have to specify types – Haskell will figure them out for you. Because they’re very different from everything people with imperative programming backgrounds are used to, they tend to cause much confusion for a long time until intuitive understanding finally develops. Parsec takes some time to get used to, but once you understand it you can drop the conventional parser generators forever. The code looks strange if you’re unfamiliar with Haskell but it’s just a matter of learning a few powerful features before you can warm up to it. Let’s view the code snippet and discuss these features one by one. Interestingly, this is one of the reasons why Haskell isn’t object oriented – it’s functions don’t need to belong to objects because the compiler and the runtime system allow us to abstract over any argument’s type and structure (contrast that to being able to abstract only over an implicit this)!
One common task all Lisp interpreters are able to perform is to print out this data structure. We’re now able to create Haskell data structures that represent Lisp’s abstract syntax trees and print them as s-expressions. In the previous section we developed a data structure capable of holding Lisp’s abstract syntax trees (a famous Lisp list). One example I can provide immediately is actually instantiating the data structure. While we could (unnecessarily) transform the above Haskell code to take up four lines, the clarity is already starting to speak for itself: you can do more with Haskell in less code that’s easier to read and maintain in the long run. Don’t let the word instance throw you off – the code above has nothing to do with instances in a conventional object oriented sense of the word. The code above uses pattern matching – the function show is defined four times. In Java we can define four versions of toString and the runtime will call the correct one depending on the instance type. If you’re confused about what the type of something is, you can always start an interpreter and ask (:t something).
We’ve now come to a point where we can get to the core of the interpreter – the code that evaluates Lisp expressions. Haskell does the same thing, however, it doesn’t limit the abstraction to types – we can get the compiler to write such switches based on any component we’re interested in abstracting! What are the types of internet marketing? For tote bag fabric the second test case I decided to go with a web application – plenty of people are writing them for money and there hasn’t been much Haskell-related work done in this area. From Business: Sierra Mills is focused on providing service and support to all levels of business throughout the entire San Francisco Bay Area. This provides the confidence among the staff as for your business. A while ago, after what now seems like eternity of flirting with Haskell articles and papers, I finally crossed the boundary between theory and practice and downloaded a Haskell compiler. While the jury’s still out how well Haskell performs in other domains (I’m just starting my web application project) I hope this article sparks your interest in this beautiful language and explains some things programmers new to Haskell often find confusing.
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