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October 10, 2020

Clojure Goodness: Formatting With Java Format String

In Clojure we can format a string using Common Lisp format syntax or the Java format string syntax. In the post we will look at the how we can use the Java format string syntax. We must use the format function in the clojure.core namespace. The method delegates to the standard JDK String#format method. The first argument is a format string followed by one or more arguments that are used in the format string. We can look up the syntax of the format string in the Javadoc for the java.util.Formatter class.

In the following example code we use the format function with different format strings:

(ns mrhaki.string.format
  (:require [clojure.test :refer [is]])
  (:import (java.util Locale)))

;; Create new string with format string as template as first argument.
;; Following arguments are used to replace placeholders in the
;; format string.
;; Clojure will delegate to the java.lang.String#format method and
;; we can use all format string options that are defined for this method.
;; More details about the format string syntax can be found in
;; java.util.Formatter. In a REPL we can find the docs 
;; with (javadoc java.util.Formatter).
(is (= ""
       (format "https://%s/" "")))

;; Format string with argument index to refer to one argument twice.
(is (= "clojure CLOJURE"
       (format "%1$s %1$S" "clojure")))

;; Format string to define fixed result lenght of 10 characers
;; with padding to get the given length.
(is (= "   Clojure"
       (format "%10s" "Clojure")))

;; Default Locale is used to determine how locale specific 
;; formats are applied. In the following example the default
;; decimal separator is . and group separator is , as specified
;; for the Canadian Locale.
(Locale/setDefault Locale/CANADA)
(is (= "Total: 42,000.00"
       (format "Total: %,.2f", 42000.0)))

(defn format-locale
  "Format a string using String/format with a Locale parameter"
  [locale fmt & args]
  (String/format locale fmt (to-array args)))

;; We can use a different Locale to apply different specific 
;; locale formats. In the next example we use the Dutch Locale
;; and the decimal seperator is , and the group separator is ..
(is (= "Totaal: 42.000,00"
       (format-locale (Locale. "nl") "Totaal: %,.2f" 42000.0)))

Written with Clojure 1.10.1.