Welcome to the third installment of “Functional programming without feeling stupid”! I originally started to describe my own learnings about FP in general, and Clojure in particular, and soon found myself writing a kind of Clojure tutorial or introduction. It may not be as comprehensive as others out there, and I still don’t think of it as a tutorial — it’s more like a description of a process, and the documented evolution of a tool.
I wanted to use Clojure “in anger”, and found out that I was learning new and interesting stuff quickly. I wanted to share what I’ve learned in the hope that others may find it useful.
Some of the stuff I have done and described here might not be the most optimal, but I see nothing obviously wrong with my approach. Maybe you do; if that is the case, tell me about it in the comments, or contact me otherwise. But please be nice and constructive, because…
…in Part 0 I wrote about how some people may feel put off by the air of “smarter than thou” that sometimes floats around functional programming. I’m hoping to present the subject in a friendly way, because much of the techniques are not obvious to someone (like me) conditioned with a couple of decades of imperative, object-oriented programming. Not nearly as funny as Learn You a Haskell For Great Good, and not as zany as Clojure for the Brave and True — just friendly, and hopefully lucid.
xkcd 1270: Functional. Licensed under Creative Commons Attribution-Non-Commercial License. This is a company blog, so it is kind of commercial by definition. Is that a problem?
In Part 1 we played around with the Clojure REPL, and in Part 2 we started making definitions and actually got some useful results. In this third part we’re going to take a look at Clojure functions and how to use them, and create our own — because that’s what functional programming is all about.
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In this installment of “Functional programming without feeling stupid” I would like to show you how to define things in Clojure. Values and function applications are all well and good, but if we can’t give them symbolic names, we need to keep repeating them over and over.
Before we start naming things, let’s have a look at how Clojure integrates with Java. I’m assuming you are still in the REPL, or have started it again with
Track 1: “If anyone should ask / We are mated”
As it happens, Clojure’s core library is lean and focused on manipulating the data structures of the language, so many things are deferred to the underlying Java machinery as a rule. For example, mathematical computation is typically done using the static methods in the
user=> (java.lang.Math/sqrt 5.0)
As you can see, this is a function application like we have already seen in Part 1, but this time the function we are using is the
sqrt static method in the
Java 7 acquired Unicode character names, and they are accessed through
getName method in the
java.lang.Character class. This is no mean feat, since there are over 110,000 characters in the Unicode standard, and each of them has a name (although some of them are algorithmically generated). To find out the canonical character name of a Unicode character, such as the euro currency symbol, you would use the
getName static method:
user=> (java.lang.Character/getName \u20ac)
ClassCastException java.lang.Character cannot be cast to java.lang.Number user/eval703 (NO_SOURCE_FILE:1)
Hey, what’s wrong? Well, if you look up the documentation of
java.lang.Character.getName, you will find out that it takes an
int value as an argument, not a character. You can actually do this check from inside the REPL:
user=> (javadoc java.lang.Character)
The REPL doesn’t seem to do much, but you should now have a new web browser window or tab open, with the JavaDoc of the java.lang.Character class loaded up. That’s what the REPL meant when it said
Javadoc: (javadoc java-object-or-class-here)
when it started up. The
getName method does need an
int value, so let’s try something else:
user=> (java.lang.Character/getName (int \u20ac))
All right! How about another one:
user=> (java.lang.Character/getName 67)
"LATIN CAPITAL LETTER C"
Well, some say that Clojure is the new C.
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