2016-01-03
Entry tags:
two variances in Scala
scala> val s = Set("a", "b", "c")
s: scala.collection.immutable.Set[String] = Set(a, b, c)
scala> val t = s.map(_ + ":)")
t: scala.collection.immutable.Set[String] = Set(a:), b:), c:))
scala> val s = Set("a1", "a2", "a3")
s: scala.collection.immutable.Set[String] = Set(a1, a2, a3)
scala> val t = s.map(_ take 1)
t: scala.collection.immutable.Set[String] = Set(a)
scala> val u:Set[Any] = s map identity
u: Set[Any] = Set(a1, a2, a3)
scala> val v:Set[Any] = s
<console>:8: error: type mismatch;
found : scala.collection.immutable.Set[String]
required: Set[Any]
Note: String <: Any, but trait Set is invariant in type A.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
val v:Set[Any] = s
^
From a categorist's p.o.v, wtf, if we have
map, we have a covariant functor. But wow, it's "type theory", covariance here means only covariance w.r.t. subtyping. So, big deal, map with identity, no? I mean, not being a typist, I don't even understand the problem. Do you?