Reputation: 3914
This is starting to be a pretty common situation for me:
trait CoVariant[+T] {
def bar: T
}
def curried[T](x: String)(y: CoVariant[T]) = y // Not really, but for simplicity
val applied = curried("foo") _
val bar = new CoVariant[String] {
def bar = "bar"
}
applied(bar)
So I have something that's co-variant and I need to do some common stuff first and then have a generic part left unapplied. I would really like to be able to write it just like a curried function like above, but that of course yields a compile error on the last line:
- type mismatch; found : CoVariant[String] required: CoVariant[Nothing]
I seem to resort to just introducing a class just to have someplace to stick the type parameter so it can be left abstract:
trait StupidWrapper {
def apply[T](y: CoVariant[T]) : CoVariant[T]
}
and instead of the curried thing have
def notAsNice(x: String) = new StupidWrapper {
def apply[T](y: CoVariant[T]) = y
}
so now:
val applied = notAsNice("foo")
applied(bar)
compiles.
This feels stupid and I guess there is better ways?
Update:
I think I'll better concretize. What I have is this:
abstract class ParserToSeq {
def apply[T](parser: Parser[T]): Seq[T]
}
def fromTrainingData(trainingLines: Seq[String]) = new ParserToSeq {
def apply[T](p: Parser[T]) = trainingLines.map(parseAll(p, _)).map {
_ match {
case Success(wt, _) => Some(wt)
case _ => None
}
}.flatten
}
and then
val thisData = fromTrainingData(trainingLines)
lazy val wordTags = thisData(wordtagParser) // Parser[WordTag]
lazy val uniGrams = thisData(uniGramParser) // Parser[UniGram]
…
I just want to do away with the ParserToSeq and do:
def fromTrainingData[T](trainingLines: Seq[String])(p: Parser[T]) =
trainingLines.map(parseAll(p, _)).map {
_ match {
case Success(wt, _) => Some(wt)
case _ => None
}
}.flatten
Note that the T here is specific for each parser. The problem as I understand it that the T is resolved to a concrete type on the partially applied method even though the type actually applies to the second parameter list.
It seems I would like to kindof stick the declaration of the type param somewhere in the middle:
def fromTrainingData(trainingLines: Seq[String])[T](p: Parser[T])
which of course isn't valid Scala.
Upvotes: 2
Views: 102
Reputation: 21112
As you mention, scala Function
objects don't carry type parameters.
However, you can replace ParseToSeq
with a more generic ~>
that takes higher-kinded types,
trait ~>[A[_],B[_]] {
def apply[X](a : A[X]) : B[X]
}
Then your example becomes,
trait Parser[T]
trait WordTag
trait UniGram
val trainingLines: Seq[String] = ???
val wordtagParser: Parser[WordTag] = ???
val uniGramParser: Parser[UniGram] = ???
def fromTrainingData[T](trainingLines: Seq[String]) = new ~>[Parser,Seq] {
def apply[T](p: Parser[T]): Seq[T] = ???
}
val thisData = fromTrainingData(trainingLines)
lazy val wordTags = thisData(wordtagParser) // Seq[WordTag]
lazy val uniGrams = thisData(uniGramParser) // Seq[UniGram]
Upvotes: 2