Why does the call to foo(1) work in my Scala 2.11.7 repl as described below?
scala> def foo[F[_], A](fa: F[A]) = null
foo: [F[_], A](fa: F[A])Null
scala> foo(List(1))
res0: Null = null
scala> foo(1)
res1: Null = null
The parameter in my call to foo(1) is not a Type Constructor so why does the Scala repl accept it?
foo(1) works because there is an implicit conversion int2Integer to java.lang.Integer
foo(int2Integer(1))
and Integer is an instance of Comparable[Integer]
public final class Integer extends Number implements Comparable<Integer>
where we see Comparable is indeed a type constructor. For example, we can reproduce the same behaviour like so
def foo[F[_], A](fa: F[A]) = 42
trait Bar // not a type constructor
implicit def barToComparable(b: Bar): Comparable[Int] = (o: Int) => -1
val bar = new Bar {}
foo(bar) // OK due to implicit conversion barToComparable