Scala string interpolation with a pass-by-name string

Question

I'd like to pass a default string to a function and have "string interpolation" done on it in the function rather than at the call site.

For example,

def isBetween(a:Int, b:Int, 
              msg: String = s"${v} is not between ${a} and ${b}."
             )(v:Int):Either[String, Boolean] = {
  if (a <= v && v <= b) Right(true) else Left(msg)
}

This doesn't compile because none of a, b, and for sure not v are in scope when the compiler wants to do the interpolation.

The goal is to provide a default error string but allow the user to change it, if necessary. For example:

val normalBetween = isBetween(0, 100)
val customBetween = isBetween(0, 100, s"Doofus! it's gotta be ${a} <= v <= ${b} but v is ${v}!")

val result1 = normalBetween(101) // Left("101 is not between 0 and 100.")
val result2 = customBetween(101) // Left("Doofus! it's gotta be 0 <= v <= 100 but v is 101!")

I tried making msg pass-by-name; no luck.

I suppose I want something like this from the Python world:

name = 'world'
program ='python'
print('Hello {name}!This is{program}.'.format(name=name, program=program))

Any suggestions?

Answer 1

As @LuisMiguelMejíaSuárez suggested in the comment, you can just use java's string formatting:

def isBetween(a: Int, b: Int, msg: String = "%%d is not between %d and %d.")(v: Int): Either[String, Boolean] = {
  if (a <= v && v <= b) Right(true) else Left(msg.format(a, b).format(v))
}

def normalBetween: Int => Either[String, Boolean] = isBetween(0, 100)
def customBetween: Int => Either[String, Boolean] = isBetween(0, 100, "Doofus! it's gotta be %d <= v <= %d but v is %%d!")

val result1 = normalBetween(101) // Left("101 is not between 0 and 100.")
val result2 = customBetween(101) // Left("Doofus! it's gotta be 0 <= v <= 100 but v is 101!")
println(result1)
println(result2)

The result will be as expected. Code run at Scastie. If you are taking this approach, and your scenario inn reality is more complex than the given example, you can use named parameters in this string. More can be read about it at Named placeholders in string formatting, How to format message with argument names instead of numbers?, and many more articles.