So I’m starting to learn Clojure, using Stuart Halloway’s book Programming Clojure as a guide. It’s a steep learning curve for me as I’ve only ever done procedural and object-oriented programming, save for a brief dalliance with Scheme in a programming course I took in graduate school.

I get the basic concepts of functional programming (immutable data, functions without side effects, etc.) but am nowhere near the point where this functional mindset comes naturally to me. I still want to break things down into objects, with methods that carry out actions step-by-step.

One of the the things about Clojure that seems to be tripping me up a lot is the syntax surrounding the ns, use and require special forms. For example, one of the first references to use occurs on p. 19 of Programming Clojure, and it looks like this:

A bit later, on p. 38, he reintroduces use, but this time the example looks like this:

Now, I get that in the first example, he’s saying “only use the str-join function from the clojure.contrib.str-utils library,” while in the second example he’s using everything. What I don’t get is the differing syntax, using a quoted vector in the first example, and a quoted list in the second. It sort-of makes me think that this should work too:

but of course it doesn’t, and I get a mostly useless error message in the REPL. Now, if you’re an experienced Lisp, Scheme or Clojure programmer, you probably understand why this last example is wrong and you really don’t understand how I could possibly think that could be right, but I’m just not there yet. But I’m working on it!

Another thing that has hindered my progress a bit is the way that Clojure’s standard library and its API documentation are organized. Take the clojure.core library for example: it contains several hundred functions, and unless you already know the name of the function you’re looking for, tracking down just the right function for what you need is like looking for a needle in a haystack. For example, a little program I was playing with yesterday dealt with a list of maps, like so:

Now what I wanted was to convert this list of maps into a single map, where the names of the relation types were the keys and the values were the relation types themselves; i.e. a result like this:

After looking through the API documentation for clojure.core I decided there wasn’t any function that would perform this kind of transformation directly (or at least not that I could tell). I also determined (I think) that there’s only one function, hash-map, that will produce a map from its inputs; everything else seems to produce a list/sequence. So anyways, I ended up going with this approach:

and it works, but I’m looking at it and wondering, is this the best way to do this? Is there maybe a more efficient approach than first constructing a list of the type names, then interleaving those with the types to produce yet another list, and finally using hash-map to convert it into a map?

Having said all that, the challenge of learning this new (to me) approach to programming is fun, and I anticipate that it’s going to affect how I think about programs that I write in the languages that I already know well, like Ruby and Java. I’m also excited about finally having what feels like a more “practical” Lisp to play with, one that I can use with existing Java libraries that I work with every day.