Type Parameter Variance

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In Java, a List<String> is not a List<CharSequence> even though a String is a CharSequence.

void m1(java.util.List<CharSequence> s) {}
void m2(java.util.List<String> s) { m1(s); } // bzzt!

Java's type parameters are always invariant, but would be nice if they were covariant^[9].
Covariance doesn't mix well with side-effects^[10]

But we aren't married to them anymore! We can have covariance, at a small cost — since we are still dating them. We use + to denote covariance^[11]

scala> trait List[+A] { val head: A; val tail: List[A] }

scala> val x = new List[String] { val head = "abc"; val tail = null }

scala> val y: List[CharSequence] = x

^[9] This is not entirely true in that a weak form of covariance can be represented with T extends U and contra-variance with T super U.

^[10] For example, side-effecting on a covariant Java array results in an ArrayStoreException.

^[11] And - to denote contravariance; another matter.