dave yarwood
A character utility library for Clojure


October 14, 2014


(Note: some of the supplementary Unicode characters on this page may not display properly if you don’t have CJK fonts installed.)

So, I’ve been kicking this idea around for a while, and I’ve finally decided to try my hand at writing a library of character- and Unicode-related utility functions for Clojure. I’ve noticed that for whatever reason, Clojure in its current state happens to lack a character utility library. I’m sure this is at least partially due to how easy it is to just use Java inter-op to call methods from Java’s Character class. For example:

(Character/toUpperCase )
;=> \Ü

But I have found Java’s Character class to be a little unwieldy. For example, this is the function I ended up writing in order to determine whether a given character is a punctuation character according to the Unicode standard:

(defn punctuation?
  (contains? #{Character/CONNECTOR_PUNCTUATION, Character/DASH_PUNCTUATION,
               Character/START_PUNCTUATION, Character/END_PUNCTUATION,
             (Character/getType (code-point-of ch))))

Complicating matters is the inherent difficulty of working with supplementary characters in Java. For those who might not be familiar: Java characters (which are what Clojure character literals refer to) are 16-bit, which allows them to store a code point with a value between 0 and 65,535 (inclusive). This is a huge amount of characters, and most of the text you see anytime you use a computer, especially in the Western world, consists of characters in this range, which is called the Basic Multilingual Plane. This vast collection of characters encompasses almost all modern languages, including a ton of Chinese, Japanese and Korean (CJK) characters, as well as a variety of other goodies. With the expansion of available Unicode characters, at some point it became necessary for additional characters to be represented in 32 bits. Because Java characters could only hold 16 bits, the solution was to represent these supplementary characters as pairs of “surrogates”: combining just the right two surrogate characters in a string creates a particular character in one of the Supplementary Planes, i.e. characters with Unicode code points between 65,536 and 1,114,111 (inclusive). When your OS displays a string containing a valid surrogate pair, you will see the corresponding supplementary character, assuming that you have the appropriate font installed.

Naturally, working with supplementary characters in Java (and by extension, Clojure) can get a little bit complicated. The methods in Java’s Character class tend to accept either a character literal or a code point as an argument, allowing you to call some methods on supplementary characters by supplying their code points as integers. Clojure’s facilities for dealing with characters are essentially limited to methods dealing with Java character literals, i.e. only characters in the Basic Multilingual Plane. The char function in clojure.core, which returns the character at a given Unicode code point, will throw an exception if you give it any number greater than 65535:

(char 20154) ; BMP character
;=> \人

(char 167122) ; supplementary character
;=> IllegalArgumentException Value out of range for char: 167122  clojure.lang.RT.charCast (RT.java:962)

One of my favorite functions that I came up with for this library is (name subject to change) char', which is like clojure.core/char, except that it knows what to do with supplementary characters:

(require '[djy.char :as char])

user=> (char/char' 128570)
;=> "😺"

The inability of supplemental “characters” to be represented as individual characters in Java and Clojure can potentially cause problems if you’re working with text that might contain supplementary characters. Let’s say you have a string containing a supplementary character:

"This character looks fun to write: 𣲷"

Using Clojure’s seq function to represent this string as a sequence of characters yields this:

(seq "This character looks fun to write: 𣲷")
;=> (\T \h \i \s \space \c \h \a \r \a \c \t \e \r \space \l \o \o \k \s \space \f \u \n \space \t \o \space \w \r \i \t \e \: \space \? \?)

Notice that our supplementary character becomes two surrogate characters (represented in my REPL as question marks; surrogate characters are technically not really characters, just code points). Imagine that you want to inspect some text from a Chinese news article and get an accurate character count. For all you know, there could be some supplementary characters in there, and if that’s the case, then your character count will be inaccurate if all you’re doing is evaluating the size of the string in characters. This inspired me to write a function called char-seq, which behaves just like seq, but if it encounters a supplementary character, it represents it as one string rather than two characters:

(char/char-seq "This character looks fun to write: 𣲷")
;=> (\T \h \i \s \space \c \h \a \r \a \c \t \e \r \space \l \o \o \k \s \space \f \u \n \space \t \o \space \w \r \i \t \e \: \space "𣲷")

That’s just a small taste of what I’ve been working on. I’m currently proposing this to the Clojure development group either as a contrib library or as a part of Clojure itself (i.e. a “clojure.char” namespace). There’s a discussion going on the Clojure dev Google group, so feel free to join in the discussion if you have any feedback or suggestions. The library, still a work in progress, is available here on GitHub.