How do scala implicit values work?

Question

This is not a question about implicit in general - I know how it works. It's about one specific construct. Here comes a quite common patten in myBatis for Scala:

manager.readOnly { implicit session => {
    //Code block: Do some DB operations in session
    }
}

Definition of readOnly can be found here: https://github.com/mybatis/scala/blob/master/mybatis-scala-core/src/main/scala/org/mybatis/scala/session/SessionManager.scala

How I read it: Call a readOnly method on manager with its argument being a function that takes session as an argument.

My question is: Where implicit session value is taken from? Which context? I do not have to define any session object by myself.

vertti vertti · Accepted Answer · 2014-02-28T17:54:22

Implicit parameters provide a way to allow parameters of a method to be "found". This means that if a parameter value is not supplied then the compiler will search for an "implicit" value defined within scope according to resolution rules:

Variable has to be marked implicit to be considered as a candidate
Explicit definitions override implicits
Local scope is looked first
Classes companion object is searched if it exists

Scala documentation puts it this way:

First, eligible are all identifiers x that can be accessed at the point of the method call without a prefix and that denote an implicit definition or an implicit parameter. Second, eligible are also all members of companion modules of the implicit parameter’s type that are labeled implicit.

It seems there's also a bunch of edge cases and not a whole lot of good documentation so I don't encourage to go crazy with implicits or you might be in for a surprise.

How do scala implicit values work?

2 Answers