Parse an f-style interpolated string to recover arguments - scala

Question

What's the best way to create the complement for an f-style string interpolation, i.e. a way to parse the arguments/numbers in a formatted string?
E.g.
val format = "frame%04d.png"
val i = 123
val out = f"frame$i%04d.png"
assert(out == "frame0123.png")
def parse(s: String, pat: String): Int = ???
val j = parse(out, format)
assert(j == 123)
I know I can manually construct a reg-exp, I just wonder if there is a general approach with f-style interpolated strings.
I was hoping for something simple like:
val f"frame$j%04d.png" = out
But
error: macro method f is not a case class, nor does it have an unapply/unapplySeq member

Answer 1

I found this nice post about using interpolators as pattern matchers so this is starting point for you:
object StringMatcher {
val ZeroFillDec = "%0(\\d+)d(.*)".r
val SimpleDec = "%(\\d+)d(.*)".r
val String = "%s(.*)".r
def patternize(part: String) = part match {
case ZeroFillDec(len, rest) => f"(\\d{${len.toInt}})$rest"
case SimpleDec(len, rest) => f"(\\d{1,${len.toInt}})$rest"
case String(rest) => f"(.*)$rest"
case rest => f"(.*)$rest"
}
}
implicit class StringMatcher(sc: StringContext) {
import StringMatcher.patternize
object pat {
def unapplySeq(s: String): Option[Seq[String]] = {
val re = (sc.parts.head ++ (sc.parts.tail map patternize)).mkString.r
re.unapplySeq(s)
}
}
}
Using that, code
val out = "frame0123_023.png"
val pat"frame$j%04d_$i%03d.png" = out
assigns
j: String = 0123
i: String = 023