String interpolation in Scala

When I tried Scala for the first time, one of the first missing language features I noticed was string interpolation (you guessed it, I was using Ruby at the time). Of course, this is a small convenience rather than a major language feature and usually String.format and java.util.Formatter are pretty good substitutes. Still, string interpolation comes now and then in Scala discussions and one has to wonder if Scala's powerful language extension facilities can emulate a feature like this.

It turns out you can get reasonably close:

trait InterpolationContext {
  class InterpolatedString(val s: String) {

    def i = interpolate(s)
  }

  implicit def str2interp(s: String) = new InterpolatedString(s)

  def interpolate(s: String) = {
    val sb = new StringBuilder
    var prev = 0
    for (m <- "\\$\\{.+?\\}".r.findAllIn(s).matchData) {
      sb.append(s.substring(prev, m.start))
      val matchString = m.matched
      var identifier = matchString.substring(2, matchString.length - 1)
      try {
        val method = this.getClass.getMethod(identifier)
        sb.append(method.invoke(this))
      } catch {
        case _: NoSuchMethodException =>
        sb.append(matchString)
      }
      prev = m.end
    }
    sb.append(s.substring(prev, s.length))
    sb.toString
  }
}

object Test extends InterpolationContext {
  val a = "3"
  def main(args: Array[String]) {
    println("expanded: ${a}; not expanded: ${b}; more text".i)
  }
}

How does this work? If you call the i method on a String, it will force implicit conversion to a InterpolatedString, similar to how the r method works for converting to a Regex. The interpolate method uses reflection to find a method with the name equal to the identifier delimited by "${" and "}". This works both for vals and defs due to Scala's adhering to the uniform access principle. If the delimited string is not a valid identifier or a parameterless method with this name doesn't exist, it is not substituted, but stays as it is.

How do you use it? Just extend or mix in the InterpolationContext trait in the class where you want this to work, and then call i on the Strings where you want interpolation to work. The InterpolationContext trait serves two purposes- first of all, it imports the implicit conversion to the interpolated string, and second it provides access to the methods of the current object via reflection.

The limitations of this method are that interpolation only works on member vals and defs of the current object only. I rather like this, because you know you can't accidentally construct a String from user input in an html form like "Name: ${System.exec(\"echo echo pwned >> .bashrc\")}". Also, interpolation doesn't work for private members as well as local variables. Finally, you have to both mix in or extend the trait and call a method on every String (even though it's a one-letter method). This is not too bad, because you can control the scope where interpolation works and therefore avoid and debug nasty surprises easier.

I don't see this as a pattern, which can be used in real-life projects, and I don't see the use of implicits here worthy of emulation. Nevertheless, trying to copy features from other languages is instructive about the limitations of your current language of choice and the tradeoffs you get for these features. Last but not least- even if it's a gimmick feature, implementing it was fun.

Scala closures as poor man's mocks

Groovy has this feature that you can use a closure whenever you need an interface with one method only. A class implementing the interface is automatically created, and the closure provides the implementation of this single method. This process is called closure coercion and is particularly convenient to make tests readable and concise.

I'm not yet sure about the relative advantages of such code everywhere, since there might be ambiguities or the intent might be obscured. Tests usually contain a lot of boilerplate, though, so I'm all for making them more concise. Except for readability, people would be more likely to write a test if it doesn't take much effort to create yet another trivial mock.

Who tests your tests though? You don't need to answer. Still, the page about implementing interfaces in Groovy warns of this trap: "Be careful that you don't accidentally define the map with { }". Type safety could help sometimes when writing tests. Or at least that's a very good excuse to try to emulate this Groovy feature in Scala.

In Scala there are implicit conversions, which help define a controlled way to coerce a value to a different type. We cannot automatically convert all closures to all possible single-method interfaces in scope, but we can choose several we are using especially often (as it happens in tests). For instance, if we want to use a mock of java.lang.Readable often enough, we could define the following implicit conversion:

import java.nio.CharBuffer

implicit def fun2readable(f: Function1[CharBuffer,Int]): Readable =
  new Readable { def read(cb: CharBuffer) = f(cb) }

So now everywhere we need a Readable, we might just drop in the appropriate closure instead (technically, this is not a mock, but a stub):

val readable: Readable = {cb: CharBuffer => cb.put("12 34"); 5}
val s = new java.util.Scanner(readable)
assert (s.nextInt == 12)

Compare this with the Groovy version:

def readable = { it.put("12 34"); 5 } as Readable
def s = new Scanner(readable)
assert s.nextInt() == 12

In the Scala version, we are explicitly defining the type of the variable in order to force the implicit conversion. In methods, where Readable is expected as an argument, explicitly naming the type will not be necessary, whereas in Groovy you always need to coerce using "as Readable":

val s = new java.util.Scanner({cb: CharBuffer => cb.put("12 34"); 5})

So in Scala you're trading some verbosity up front for some conciseness in all of the code where the implicit conversion is visible. Furthermore, the type checker guarantees that only closures with the correct signature are converted to the interface in question (i.e. with an input of CharBuffer and output of Int) and will flag a compile-time error otherwise.

The reader might be tempted to also emulate map coercion, but beware of an unexpected difficulty: it's not easy to define both a closure (with one argument) and map coercion. Since a Map in scala is a function and due to type erasure, the compiler won't know whether to substitute the Map or the Function!

Can you generate these implicits automatically without defining them? Maybe with a Scala compiler plugin, but then you lose some of the transparency of knowing which implicits you've defined.