How to pass implicit parameter explicitly? - scala

Question

If we define the following function:
def methodWithImplicit(explicit: String)(implicit imp: String) = {
println(explicit + imp)
}
we can call it as follows:
methodWithImplicit("abc")("efg") //abc - explicit, efg - imp
And it works fine. Now consider the following TypeClass:
trait MyTypeClass[T] {
def accept(t: T): T
}
which is going to be used inside extractor object:
object TestExtractor {
def unapply(str: String)(implicit myTypeClass: MyTypeClass[String]): Option[String] =
if (!str.isEmpty)
Some(myTypeClass.accept(str))
else
None
}
So if we use it as follows:
implicit val myTypeClass:MyTypeClass[String] = new MyTypeClass[String] {
override def accept(t: String): Unit = t
}
"123" match {
case TestExtractor(str) => println(str)
}
It works ok. But how to pass the parameter explicitly when using with pattern matching? I tried
"123" match {
case TestExtractor(str)(myTypeClass) => println(str) //compile error
}
and
"123" match {
case TestExtractor(myTypeClass)(str) => println(str) //compile error
}
But it does not compile.

Answer 1

Since the left hand side seems to accept essentially nothing but trees built from stable identifiers, constant literals, and lower-case letters for variable names, I don't see any way to get closer to the desired syntax than this:
val `TestExtractor(myTypeClass)` = TestExtractor(myTypeClass)
"hello" match {
case `TestExtractor(myTypeClass)`(str) => println(str)
}
This of course requires that you define the weirdly named value TestExtractor(myTypeClass) (in backticks) right before the match-case, so you can use it as a single symbol.
Full code:
trait MyTypeClass[T] {
def accept(t: T): T
}
object TestExtractor { outer =>
def unapply(str: String)(implicit myTypeClass: MyTypeClass[String]): Option[String] =
if (!str.isEmpty)
Some(myTypeClass.accept(str))
else
None
class ExplicitTestExtractor(tc: MyTypeClass[String]) {
def unapply(t: String) = outer.unapply(t)(tc)
}
def apply(tc: MyTypeClass[String]): ExplicitTestExtractor =
new ExplicitTestExtractor(tc)
}
implicit val myTypeClass:MyTypeClass[String] = new MyTypeClass[String] {
override def accept(t: String): String = t.toUpperCase
}
val `TestExtractor(myTypeClass)` = TestExtractor(myTypeClass)
"hello" match {
case `TestExtractor(myTypeClass)`(str) => println(str)
}