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下面是用Scala和Clojure编写的用于在Strings中简单替换模板的函数。每个函数的输入是String，其中包含{key}形式的模板以及从符号/关键字到替换值的映射。Scala和Clojure中的简单字符串模板替换
例如：
斯卡拉：
replaceTemplates("This is a {test}", Map('test -> "game"))
的Clojure：
(replace-templates "This is a {test}" {:test "game"})
将返回"This is a game"。
输入地图使用符号/关键字，因此我不必处理字符串中的模板包含大括号的情况。
不幸的是，算法效率不高。
这里是Scala代码：
def replaceTemplates(text: String,
templates: Map[Symbol, String]): String = {
val builder = new StringBuilder(text)
@tailrec
def loop(key: String,
keyLength: Int,
value: String): StringBuilder = {
val index = builder.lastIndexOf(key)
if (index < 0) builder
else {
builder.replace(index, index + keyLength, value)
loop(key, keyLength, value)
}
}
templates.foreach {
case (key, value) =>
val template = "{" + key.name + "}"
loop(template, template.length, value)
}
builder.toString
}
和这里是Clojure的代码：
(defn replace-templates
"Return a String with each occurrence of a substring of the form {key}
replaced with the corresponding value from a map parameter.
@param str the String in which to do the replacements
@param m a map of keyword->value"
[text m]
(let [sb (StringBuilder. text)]
(letfn [(replace-all [key key-length value]
(let [index (.lastIndexOf sb key)]
(if (< index 0)
sb
(do
(.replace sb index (+ index key-length) value)
(recur key key-length value)))))]
(doseq [[key value] m]
(let [template (str "{" (name key) "}")]
(replace-all template (count template) value))))
(.toString sb)))
这里是一个测试用例（Scala代码）：
replaceTemplates("""
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
elit nisi, egestas et tincidunt eget, {foo} mattis non erat. Aenean ut
elit in odio vehicula facilisis. Vestibulum quis elit vel nulla
interdum facilisis ut eu sapien. Nullam cursus fermentum
sollicitudin. Donec non congue augue. {bar} Vestibulum et magna quis
arcu ultricies consectetur auctor vitae urna. Fusce hendrerit
facilisis volutpat. Ut lectus augue, mattis {baz} venenatis {foo}
lobortis sed, varius eu massa. Ut sit amet nunc quis velit hendrerit
bibendum in eget nibh. Cras blandit nibh in odio suscipit eget aliquet
tortor placerat. In tempor ullamcorper mi. Quisque egestas, metus eu
venenatis pulvinar, sem urna blandit mi, in lobortis augue sem ut
dolor. Sed in {bar} neque sapien, vitae lacinia arcu. Phasellus mollis
blandit commodo.
""", Map('foo -> "HELLO", 'bar -> "GOODBYE", 'baz -> "FORTY-TWO"))
和输出：
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
elit nisi, egestas et tincidunt eget, HELLO mattis non erat. Aenean ut
elit in odio vehicula facilisis. Vestibulum quis elit vel nulla
interdum facilisis ut eu sapien. Nullam cursus fermentum
sollicitudin. Donec non congue augue. GOODBYE Vestibulum et magna quis
arcu ultricies consectetur auctor vitae urna. Fusce hendrerit
facilisis volutpat. Ut lectus augue, mattis FORTY-TWO venenatis HELLO
lobortis sed, varius eu massa. Ut sit amet nunc quis velit hendrerit
bibendum in eget nibh. Cras blandit nibh in odio suscipit eget aliquet
tortor placerat. In tempor ullamcorper mi. Quisque egestas, metus eu
venenatis pulvinar, sem urna blandit mi, in lobortis augue sem ut
dolor. Sed in GOODBYE neque sapien, vitae lacinia arcu. Phasellus mollis
blandit commodo.
该算法遍历输入映射，并且对于每个对，在输入String中进行替换，临时保存在StringBuilder中。对于每个键/值对，我们搜索键的最后一次出现（用大括号括起来），并将其替换为该值，直到不再出现为止。
如果我们在StringBuilder中使用.lastIndexOf与.indexOf是否会产生性能差异？
算法如何改进？有没有一种更习惯的方式来编写Scala和/或Clojure代码？
UPDATE：请参阅我的follow-up。
更新2：这是一个更好的Scala实现; O（n）中的字符串的长度。请注意，我根据几个人的建议将Map修改为[String, String]而不是[Symbol, String]。（感谢mikera，kotarak）：
/**
* Replace templates of the form {key} in the input String with values from the Map.
*
* @param text the String in which to do the replacements
* @param templates a Map from Symbol (key) to value
* @returns the String with all occurrences of the templates replaced by their values
*/
def replaceTemplates(text: String,
templates: Map[String, String]): String = {
val builder = new StringBuilder
val textLength = text.length
@tailrec
def loop(text: String): String = {
if (text.length == 0) builder.toString
else if (text.startsWith("{")) {
val brace = text.indexOf("}")
if (brace < 0) builder.append(text).toString
else {
val replacement = templates.get(text.substring(1, brace)).orNull
if (replacement != null) {
builder.append(replacement)
loop(text.substring(brace + 1))
} else {
builder.append("{")
loop(text.substring(1))
}
}
} else {
val brace = text.indexOf("{")
if (brace < 0) builder.append(text).toString
else {
builder.append(text.substring(0, brace))
loop(text.substring(brace))
}
}
}
loop(text)
}
更新3：这里有一组Clojure的测试用例（Scala的版本作为一个练习:-)）：
(use 'clojure.test)
(deftest test-replace-templates
(is (= ; No templates
(replace-templates "this is a test" {:foo "FOO"})
"this is a test"))
(is (= ; One simple template
(replace-templates "this is a {foo} test" {:foo "FOO"})
"this is a FOO test"))
(is (= ; Two templates, second at end of input string
(replace-templates "this is a {foo} test {bar}" {:foo "FOO" :bar "BAR"})
"this is a FOO test BAR"))
(is (= ; Two templates
(replace-templates "this is a {foo} test {bar} 42" {:foo "FOO" :bar "BAR"})
"this is a FOO test BAR 42"))
(is (= ; Second brace-enclosed item is NOT a template
(replace-templates "this is a {foo} test {baz} 42" {:foo "FOO" :bar "BAR"})
"this is a FOO test {baz} 42"))
(is (= ; Second item is not a template (no closing brace)
(replace-templates "this is a {foo} test {bar" {:foo "FOO" :bar "BAR"})
"this is a FOO test {bar"))
(is (= ; First item is enclosed in a non-template brace-pair
(replace-templates "this is {a {foo} test} {bar" {:foo "FOO" :bar "BAR"})
"this is {a FOO test} {bar")))
(run-tests)
的关键是在编译时已知？如果是这样，这太复杂了 – 2011-05-24 12:26:27
@Kim Stebel：这是怎么回事？我如何改进它？ – Ralph 2011-05-24 12:32:33
@ralph从clojure.contrib.strint看['<<']（http://clojure.github.com/clojure-contrib/strint-api.html#clojure.contrib.strint/%3C%3C） '。我认为它也转移到了新的贡献。不过，这只是编译时间。 – kotarak 2011-05-24 12:41:40