Meaning of additional colon in Scala class parametrization

Question

What does [A : Manifest : WireFormat] mean in the following code? It's from com.nicta.scoobi.TextInput (available on github). It doesn't seem to be any of the usual type bounds.

 def fromDelimitedTextFile[A : Manifest : WireFormat] (path: String, sep: String = "\t") (extractFn: PartialFunction[List[String], A]) : DList[A] = { val lines = fromTextFile(path) lines.flatMap { line => val fields = line.split(sep).toList if (extractFn.isDefinedAt(fields)) List(extractFn(fields)) else Nil } }

Where can I find more information about this topic?

Community · Accepted Answer · 2017-05-23 12:16:53Z

This is called a context bound. They are syntactic sugar for an implicit parameter list:

def meth[A : ContextBound1 : ContextBoundN](a: A) // ==> def meth[A](a: A)(implicit evidence: ContextBound1[A], ContextBoundN[A])

If there are multiple context bounds from 1 to N, they are all translated into the same parameter list. See this question for a more detailed explanation about how they work and for what they are useful.

To find such symbols it is useful to read the StackOverflow Scala Tutorial.

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