String pattern matching best practice

Question

Following is the code that doesn't work but it describes what I want to do.

Could you please recommend the best approach to this problem?

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   } } 

Answer 1

In terms of syntax, you can modify just a tiny bit you case statements:

case url if url.startsWith("jdbc:mysql:") => "com.mysql.jdbc.Driver" 

This simply binds the value url to the pattern expression (which is also url) and adds a guard expression with a test. That should make the code compile.

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):

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.

Answer 2

Here is an alternate way. Store all the mappings in a map and then use collectFirst method to find the match. Type signature of collectFirst is:

def TraversableOnce[A].collectFirst[B](pf: PartialFunction[A, B]): Option[B] 

Usage:

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)