What does the keyword 'implicit' mean when it's placed in front of lambda expression parameter?

Question

I have seen this kind of code many times before, most recently at scala-user mailing list:

context(GUI) { implicit ec =>
  // some code
}

context is defined as:

def context[T](ec: ExecutionContext)(block: ExecutionContext => T): Unit = { 
  ec execute { 
    block(ec) 
  } 
}

What purpose does the keeyword implicit achieve when placed in front of a lambda expression parameter?

possible duplicate: [Scala Functional Literals with Implicits](http://stackoverflow.com/questions/6326132/scala-functional-literals-with-implicits) — kiritsuku, Feb 18 '12 at 09:10
For posterity, additional detailed explanation: http://daily-scala.blogspot.com/2010/04/implicit-parameters.html — Eugene Cheipesh, Jul 13 '12 at 01:29

score 19 · Accepted Answer · answered Feb 18 '12 at 10:19

19

scala> trait Conn
defined trait Conn

scala> def ping(implicit c: Conn) = true
ping: (implicit c: Conn)Boolean

scala> def withConn[A](f: Conn => A): A = { val c = new Conn{}; f(c); /*cleanup*/ }
withConn: [A](f: Conn => A)A

scala> withConn[Boolean]( c => ping(c) )
res0: Boolean = true

scala> withConn[Boolean]{ c => implicit val c1 = c; ping }
res1: Boolean = true

scala> withConn[Boolean]( implicit c => ping )
res2: Boolean = true

The last line is essentially a shorthand for the second last.

answered Feb 18 '12 at 10:19

retronym

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1

BTW, rather than using implicit parameters to propagate the context (in this example, `Conn`) through the computation, you can use the Reader monad. https://gist.github.com/1395578 – retronym Feb 18 '12 at 10:23
2

Thanks for the gist too. Starred it. – missingfaktor Feb 18 '12 at 10:26
def withConn[A](f: Conn => A): A = { val c = new Conn{}; f(c); /*cleanup*/ this is missing an ending } :) – Cui Pengfei 崔鹏飞 May 22 '14 at 15:41

What does the keyword 'implicit' mean when it's placed in front of lambda expression parameter?

1 Answers1