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Couplet is a small library that provides support for working with Unicode characters or ‘code points’ in Clojure.

The distinguishing feature of this library is the type that represents a sequence of code points: that type is efficiently seqable and reducible, and also supports parallel fold via fork/join.

This library targets Clojure on the JVM.

Clojars Project Build Status

Dependency information


ch.gluet/couplet {:mvn/version "0.2.1"}


[ch.gluet/couplet "0.2.1"]



Require the core namespace, preferably aliasing it as cp.

(require '[couplet.core :as cp])

The central function in this library is cp/codepoints. When passed a string or other CharSequence, it returns a seqable/reducible succession of the Unicode code points contained in the string.

Code points are simply the platform integers (same as UTF-32 code units).

(seq (cp/codepoints "bird🐦"))
; => (98 105 114 100 128038)

The value proposition of cp/codepoints is the capability of treating strings as sequences of Unicode characters, as opposed to the awkward default treatment of such things in Clojure as sequences of chars, that is UTF-16 code units.

An example showing counting and (naive) reversal illustrates this difference:

(count (seq "bird🐦"))
; => 6
(count (seq (cp/codepoints "bird🐦")))
; => 5

(apply str (reverse "bird🐦"))
; => "??drib"
(cp/to-str (reverse (cp/codepoints "bird🐦")))
; => "🐦drib"

A sequence of code points can be turned back into a string with cp/to-str. This function can take a transducer as the first argument to apply an additional transformation to the inputs.

(def bee-codepoints
  (into [] (cp/codepoints "b🐝e🌻e")))

(cp/to-str bee-codepoints)
; => "b🐝e🌻e"

(cp/to-str (take-nth 2) bee-codepoints)
; => "bee"

Calling cp/codepoints without arguments returns a transducer that converts char inputs to code points. This transducer is useful when dealing with values that do not implement CharSequence, such as Java arrays.

(into [] (cp/codepoints) (char-array "bird🐦"))
; => [98 105 114 100 128038]

The function to turn an opaque code point integer back into readable string form is called cp/codepoint-str.

(run! (comp println cp/codepoint-str) (cp/codepoints "bird🐦"))
; b
; i
; r
; d
; 🐦

String inputs are always handled in a lenient, non-failing fashion. Invalid data such as isolated (unpaired) surrogates pass through untouched. Where desired, validation and sanitization can be implemented like any other transformation using existing general transformation functions.

(not-any? cp/surrogate? (cp/codepoints "broken\ud930"))
; => false, U+D930 is an isolated surrogate

(def sanitize-surrogates
  (map #(if (cp/surrogate? %) 0xFFFD %)))

(cp/to-str sanitize-surrogates (cp/codepoints "broken\ud930"))
; => "broken�"

In addition to supporting efficient reduction, code point sequences support parallel processing via clojure.core.reducers/fold.

For example, we can calculate the most frequently occurring Unicode blocks in some large input string. With fold, the work is transparently divided into tasks that are then processed in parallel. In the ideal case, this should improve performance by a factor proportional to the number of processors.

(require '[clojure.core.reducers :as r])

(defn update-frequencies [m cp]
  (update m (java.lang.Character$UnicodeBlock/of (int cp)) (fnil inc 0)))

(defn merge-frequencies
  ([] {})
  ([m1 m2] (merge-with + m1 m2)))

(let [s (cp/to-str (repeatedly 1e6 #(rand-int 0x1FFFF)))]
  (->> (cp/codepoints s)
       (r/fold 10000 merge-frequencies update-frequencies)
       (sort-by val >)
       (take 10)))

Specs for code points are covered by the predicate cp/codepoint? and the corresponding spec ::cp/codepoint. The macro cp/codepoint-in can be used to spec a code point range.

Thanks to the attached generator you can generate test data easily.

(require '[clojure.spec.alpha :as s]
         '[clojure.spec.gen.alpha :as gen])

(s/def ::fruit-n-veg (cp/codepoint-in 0x1F345 0x1F353))

(s/valid? ::fruit-n-veg 0x1F351)
; => true, it’s a peach

(cp/to-str (gen/sample (s/gen ::fruit-n-veg)))
; => "🍍🍍🍆🍅🍎🍓🍊🍌🍍🍓"

Code point literals are occasionally useful, for example when attempting to write human-readable cp/codepoint-in specs. Register a tagged literal of your choice to enable code point literals; the following snippet shows how.

(defn read-codepoint [s]
  (first (cp/codepoints s)))

(set! *data-readers* (assoc *data-readers* 'cp #'read-codepoint))

(s/valid? (cp/codepoint-in #cp "🍅", #cp "🍓")
          #cp "🍑")
; => true

Refer to the java.lang.Character Javadoc for JDK APIs that can be fruitfully combined with the functionality provided in this library.

Design goals

  • small: provide basic building blocks for working with Unicode characters, not more
  • efficient: as performant as reasonably possible in Clojure on the JVM
  • transparent: allow processing any string, no well-formedness requirement imposed, no exceptions thrown nor mangling done on ill-formed UTF-16 input

Related work

There are other solutions for the same problem, though perhaps written with different goals in mind.

Check out ICU for an extensive, mature Java library for Unicode.


Run the benchmarks with

lein jmh '{:type :quick, :format :table}'

The following is a short summary of the findings.

Broadly speaking, processing strings using code points instead of chars has no negative impact on performance. On the contrary, the performance achieved here compares favourably with that of Clojure’s own char-based string processing.

  • Reduce is faster than processing a lazy seq of code points by a factor of 3.
  • Parallel fold can be faster than reduce by a factor proportional to the number of cores.
  • Compared with Clojure strings, performance differences range from on par (reducing code points versus reducing a string) to faster by a factor of 3 (cp/to-str versus apply str) to faster by a factor of 5 (lazy seq of code points versus lazy seq of chars).

Strings support fast random access – code point seqs do not. For efficient lookup of code points by index consider a vector-of :int or Java array of int.


Copyright © 2017–2020 David Bürgin

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

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