Subject: Re: Variable typing Newsgroups: gmane.comp.lang.scala.user Date: Wednesday 11th July 2007 08:59:16 UTC (over 11 years ago) > For a var, that would be difficult, because we cannot make var > parametric. The best approximation is maybe > > scala> def bar[X] = (x:X) => foo(x) > bar: [X](X) => X > > Maybe you are consoled by the fact that in the polymorphic > lambda calculus, it would not be a problem to define *values* > of such a type (bar is a *method* in Scala) > [...] > Burak It is true that in Scala, methods can have polymorphic types but values cannot. However, with type constructor polymorphism, I think it is possible to encode values with polymorphic types. The encoding relies on the following class. // Forall[A] = "forall X, A[X]" trait Forall[A[_]] { def Apply[X]: A[X] } By definition, an instance of a polymorphic type "forall X, A[X]" must provide a value of type "A[T]" for every type "T". We can check that this is actually the case for an instance "x" of the Scala type "Forall[A]": for every "T", "x.Apply[T]" is a value of type "A[T]". As an example, we can define a value "f" that holds the polymorphic function that returns the head of a list: // val f: [X] List[X] => X = [X] (xs: List[X]) => xs.head type A[X] = List[X] => X val f: Forall[A] = new Forall[A] { def Apply[X]: A[X] = (xs: List[X]) => xs.head } Then, we can use "f" with different type instances: // val i: Int = f[Int](List(1,2)) val i: Int = f.Apply[Int].apply(List(1,2)) // val s: String = f[String](List("hello","world")) val s: String = f.Apply[String].apply(List("hello","world")) Vincent |
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