CODEWITHSUNDEEP
scala
Mazerunner72
Mazerunner72

Reputation: 651

Scala Implicit syntax in polymorphic methods

I am a Scala noob reading through a parsing library, and have reached some syntax I do not understand:

def parseA[_: P] = P("a")

val Parsed.Success(value, successIndex) = parse("a", parseA(_))

I want to be able to combine these lines into one, ie

val Parsed.Success(value, successIndex) = parse("a", P("a"))

but this gives a compile error:

Error:(8, 61) overloaded method value P with alternatives:
  [T](t: fastparse.P[T])(implicit name: sourcecode.Name, implicit ctx: fastparse.P[_])fastparse.P[T] <and>
  => fastparse.ParsingRun.type
 cannot be applied to (String)
Error occurred in an application involving default arguments.
    val Parsed.Success(value, successIndex) = parse(source, P("a"))

How should this line be written? And can you name the syntax concepts involved to maximise my learning?

Upvotes: 0

Views: 251

Answers (1)

Luis Miguel Mej&#237;a Su&#225;rez
Luis Miguel Mej&#237;a Su&#225;rez

Reputation: 22840

_: P is the same as (implicit ctx: P[_]), that means that method is asking for an implicit parameter of type P[_] (the underscore means that it does not care for the inner type. See What are all the uses of an underscore in Scala?).
P("a") is calling this method, which requires such implicit in scope, and that is why in your second example it fails to compile, because it did not find the implicit parameter.
The features sued here are implicits, existential types & macros...

All of them are very advanced techniques. If you are just starting, I would suggest to leave them for latter.
Implicits are very important and useful, I would start from there, but first make sure you feel comfortable with "normal" Scala (whatever that means).

For the second question, I think this should work.

def program[_: P] = parse("a", P("a"))
val Parsed.Success(value, successIndex) = program

Upvotes: 1

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