scala 字符串模式匹配最佳实践
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String pattern matching best practice
提问by Nikita Volkov
Following is the code that doesn't work but it describes what I want to do.
以下是不起作用的代码，但它描述了我想要做什么。
Could you please recommend the best approach to this problem?
您能否推荐解决此问题的最佳方法？
def resolveDriver(url: String) = {
url match {
case url.startsWith("jdbc:mysql:") => "com.mysql.jdbc.Driver"
case url.startsWith("jdbc:postgresql:") => "org.postgresql.Driver"
case url.startsWith("jdbc:h2:") => "org.h2.Driver"
case url.startsWith("jdbc:hsqldb:") => "org.hsqldb.jdbcDriver"
case _ => throw new IllegalArgumentException
}
}
回答by huynhjl
In terms of syntax, you can modify just a tiny bit you case statements:
在语法方面，你可以修改一点你的 case 语句：
case url if url.startsWith("jdbc:mysql:") => "com.mysql.jdbc.Driver"
This simply binds the value urlto the pattern expression (which is also url) and adds a guard expression with a test. That should make the code compile.
这只是将值绑定url到模式表达式（也是url）并添加一个带有测试的保护表达式。这应该使代码编译。
To make it a little bit more scala-like, you can return an Option[String] (I removed a couple clause since it's just for illustration):
为了让它更像 Scala，你可以返回一个 Option[String] （我删除了几个子句，因为它只是为了说明）：
def resolveDriver(url: String) = url match {
case u if u.startsWith("jdbc:mysql:") => Some("com.mysql.jdbc.Driver")
case u if u.startsWith("jdbc:postgresql:") => Some("org.postgresql.Driver")
case _ => None
}
That is unless you want to manage exceptions.
除非您想管理异常。
回答by missingfaktor
Here is an alternate way. Store all the mappings in a map and then use collectFirstmethod to find the match. Type signature of collectFirstis:
这是另一种方法。将所有映射存储在一个映射中，然后使用collectFirst方法查找匹配项。的类型签名collectFirst是：
def TraversableOnce[A].collectFirst[B](pf: PartialFunction[A, B]): Option[B]
Usage:
用法：
scala> val urlMappings = Map("jdbc:mysql:" -> "com.mysql.jdbc.Driver", "jdbc:postgresql:" -> "org.postgresql.Driver")
urlMappings: scala.collection.immutable.Map[java.lang.String,java.lang.String] = Map(jdbc:mysql: -> com.mysql.jdbc.Drive
r, jdbc:postgresql: -> org.postgresql.Driver)
scala> val url = "jdbc:mysql:somestuff"
url: java.lang.String = jdbc:mysql:somestuff
scala> urlMappings collectFirst { case(k, v) if url startsWith k => v }
res1: Option[java.lang.String] = Some(com.mysql.jdbc.Driver)
回答by Xavier Guihot
Starting Scala 2.13, it's possible to pattern match a Strings by unapplying a string interpolator:
开始Scala 2.13，可以String通过不应用字符串插值器来模式匹配s ：
val s"jdbc:$dialect:$rest" = "jdbc:mysql:whatever"
// dialect: String = "mysql"
// rest: String = "whatever"
Then in our case, it's just a matter of mapping the extracted value (the sql dialect) to the appropriate driver using a Map:
然后在我们的例子中，只需使用以下命令将提取的值（sql 方言）映射到适当的驱动程序Map：
val drivers = Map(
"postgresql" -> "org.postgresql.Driver",
"mysql" -> "com.mysql.jdbc.Driver",
"h2" -> "org.h2.Driver"
)
val driver = drivers(dialect)
// driver: String = "com.mysql.jdbc.Driver"
If you are expecting malformed inputs, you can also use a match statement:
如果您期待格式错误的输入，您还可以使用 match 语句：
"jdbc:postgresql:something" match {
case s"jdbc:$dialect:$rest" => Some(dialect)
case _ => None
}
// Option[String] = Some("postgresql")
回答by Volty De Qua
import PartialFunction._ // condOpt
val drivers = Map(
"mysql" -> "mysql driver",
"h2" -> "h2 driver"
// ...
)
val pattern = "^jdbc:(\w+):.*".r
def resolveDriver(url: String) = condOpt(url) {
case pattern(dbms) => drivers.get(dbms)
}.flatten.getOrElse(throw new IllegalArgumentException)
