Eran Medan Eran Medan - 29 days ago 7
Scala Question

String interpolation in Scala 2.10 - How to interpolate a String variable?

String interpolation is available in Scala starting Scala 2.10

This is the basic example

val name = "World" //> name : String = World
val message = s"Hello $name" //> message : String = Hello World


I was wondering if there is a way to do dynamic interpolation, e.g. the following (doesn't compile, just for illustration purposes)

val name = "World" //> name : String = World
val template = "Hello $name" //> template : String = Hello $name
//just for illustration:
val message = s(template) //> doesn't compile (not found: value s)



  1. Is there a way to "dynamically" evaluate a String like that? (or is it inherently wrong / not possible)

  2. And what is
    s
    exactly? it's not a method def (apparently it is a method on
    StringContext
    ), and not an object (if it was, it would have thrown a different compile error than not found I think)





Answer

s is actually a method on StringContext (or something which can be implicitly converted from StringContext). When you write

whatever"Here is text $identifier and more text"

the compiler desugars it into

StringContext("Here is text ", " and more text").whatever(identifier)

By default, StringContext gives you s, f, and raw* methods.

As you can see, the compiler itself picks out the name and gives it to the method. Since this happens at compile time, you can't sensibly do it dynamically--the compiler doesn't have information about variable names at runtime.

You can use vars, however, so you can swap in values that you want. And the default s method just calls toString (as you'd expect) so you can play games like

class PrintCounter {
  var i = 0
  override def toString = { val ans = i.toString; i += 1; ans }
}

val pc = new PrintCounter
def pr[A](a: A) { println(s"$pc: $a") }
scala> List("salmon","herring").foreach(pr)
1: salmon
2: herring

(0 was already called by the REPL in this example).

That's about the best you can do.

*raw is broken and isn't slated to be fixed until 2.10.1; only text before a variable is actually raw (no escape processing). So hold off on using that one until 2.10.1 is out, or look at the source code and define your own. By default, there is no escape processing, so defining your own is pretty easy.

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