Lazy function arguments?

Question

How to make "areYouLazy" function evaluate "string" only once the best possible way?

def areYouLazy(string: => String) = {
    string
    string
  }

areYouLazy {
  println("Generating a string")
  "string"
}

I don't think it's possible, because your string is not a variable. — Ashalynd
– Ashalynd, Commented Oct 4, 2014 at 16:03
You can assign it to a variable in areYouLazy, like val notLazyAnymore = string; string. — Ende Neu
– Ende Neu, Commented Oct 4, 2014 at 16:04

Jean Logeart · Accepted Answer · 2014-10-04 16:17:01Z

7

Call-by-name arguments are executed everytime you access them.

To avoid multiple executions, you can simply use a lazy cache value that is executed only the first time it is accessed:

def areYouLazy(string: => String) = {
  lazy val cache = string
  cache  // executed
  cache  // simply access the stored value
}

edited Oct 4, 2014 at 16:17

answered Oct 4, 2014 at 16:06

Jean Logeart

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2 Comments

Anton Kuzmin Over a year ago

Thank you! I ended up doing it through a function. Seems more readable. Less names to give to variables.

private def putOnTable(getCard: () => Option[Card])(implicit table: Table) = getCard() match {     case cardOption @ Some(card) =>       table.addCard(card)        cardOption     case None =>       None   }

Rex Kerr Over a year ago

@HappyCoder - I don't understand why you think getCard isn't an extra name. Also, it's harder to pass an argument to () => A than to => A. There might be reasons to do it your way, but "more readable" and "fewer names" aren't among them.

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