Maybe in Java 17 Nov 2011

One of the more elegant concepts in a lot of functional languages is that of Maybe. Haskell drove home the magic of Maybe for me. The general idea is representing the possibility of a value. In Haskell, you’ll generally use it such that processing will continue if the value is needed and present, or short circuit if the value is needed and not present. The end result of a computation which includes a Maybe will also be a Maybe, but one representing the result of the computation.

In expression-oriented languages like Haskell, that works out very nicely, but I spend most of my time working in Java, which is decidedly statement-oriented. Concepts will sometimes move nicely between worlds, sometimes not. When I started working on Atlas recently I decided to see how well Maybe ported. I started with Nat Pryce’s maybe-java and took off from there.

Nat’s class encourages a model analogous to Haskell’s, executing within the context of a Maybe instance by passing functions into the Maybe instance, for example:

Maybe<String> name = Maybe.definitely("Brian");
Maybe<String> message = name.to(new Function<String, String>() {
    public String apply(String s)
    {
        return "hello, " + s;
    }
});
System.out.println(message.otherwise("No one here!"));

This treats the sequence of Maybe instances as an evaluation context or control flow, which works nicely in some languages, but sadly, as with most attempts to do higher-order functions in Java, it got awkward rather quickly. Part of it is purely syntactic, the syntax isn’t optimized for it, but part of it is semantic as well. Idiommatic Java uses exceptions for non-happy-path control flow, and most of the libraries which provide the main reason for using Java behave this way.

Given that, I switched from using Maybe to control evaluation to using Maybe purely to represent the possibility of a value, and things fell into place very nicely – even playing within Java’s exception idioms. Take for example this snippet:

SSHCredentials creds = space.lookup(credentialName, SSHCredentials.class)
    .otherwise(space.lookup(SSHCredentials.DEFAULT, SSHCredentials.class)
    .otherwise(new IllegalStateException("unable to locate any ssh credentials"));

In this case there may exist named credentials, if not there may exist some default credentials, and if there is neither the world explodes. In the typical case you would see either a test for existence and then use, or a fetch and check for null. Both of which are, to my mind, less clear and certainly more error prone (largely in needing to remember to check everywhere, particularly in the case of a this-or-that situation, etc).

Other bits of using Maybe extensively are not completely clear, but I am pretty confident that I will be using some evolution this flavor of Maybe in most of my Java-based code going forward.