Extracting with regular expressions

Question
1

Reading thru the Programming in Scala 4ed, in 26.7 “What’s more, every regular expression in Scala defines an extractor.”

and in the example:

val Decimal = """(-)?(\d+)(\.\d*)?""".r
val Decimal(sign, integer part, decimal part) = "-1.23"

my question is how does that unapply method get defined for each Decimal object? Thru some runtime magic?

2

Regex objects don’t have an unapply method. They use unapplySeq, which uses varargs and returns an Option[Seq[A]]. The documentation only mentions shortly in the last paragraph, that this alternative to unapply exists.

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3

Thanks for the reply.

My confusing point is how does the compiler know the decimal need to match the 3 captures. Checked a little further and it turns out the compiler does NOT.

So the match is more like:

Decimal.findMatches("-1.23") @unchecked match {
  case (sign, integerpart, decimalpart) => (sign, integerpart, decimalpart)
}

and just throw runtime error: scala.MatchError when the match fails.

4

Yes, but this is generally true for any pattern assignment, as the exhaustiveness checks only work well for statically known parts.

case class Foo(i: Int)
def foo(x: Boolean): Any = if(x) Foo(1) else ""

val Foo(y) = foo(true) // y: Int = 1

val Foo(y) = foo(false) // scala.MatchError:  (of class java.lang.String)

So if you aren’t sure at compile time, that the pattern will match, a match expression with a branch for the non-matching case is better.

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