match(5)
{
  | 4 => Console.WriteLine(4);
  | active Sum(2,x) => Console.WriteLine(x);
  | _ => Console.WriteLine("You will never see it");
}

def m() {
  ...
  def t : object = ...;
  match(t) {
    | _ is IDictionary['k,'v] => {}
    | _ is IEnumerable['k] => {}
    | _ => {}
  }
  ...
}

def m['k,'v]() {
  ...
  def t : object = ...;
  match(t) {
    | _ is IDictionary['k,'v] => {}
    | _ is IEnumerable['k] => {}
    | _ => {}
  }
  ...
}

def d = Dictionary.[int, string]();
// | d is Dictionary[TKey, TValue] => {}
Console.WriteLine(d.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<,>)));

// | d is Dictionary[int, TValue] => {}
Console.WriteLine(d.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<,>))
                  && d.GetType().GetGenericArguments()[0] == typeof(int));

А>    | _ is IDictionary['k,'v] => {}
А>

  | x is IDictionary['k,'v] => // Какой будет тип у "x"?

I>def d = Dictionary.[int, string]();
I>// | d is Dictionary[TKey, TValue] => {}
I>Console.WriteLine(d.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<,>)));

I>// | d is Dictionary[int, TValue] => {}
I>Console.WriteLine(d.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<,>))
I>                  && d.GetType().GetGenericArguments()[0] == typeof(int));
I>

def m()
{
  ...
  def t : object = ...;
  match(d.GetType().GetGenericTypeDefinition())
  {
    | x when x.IsAssignableFrom(typeof(IDictionary[_,_])) => {}
    | x when x.IsAssignableFrom(typeof(IEnumerable[_]))  => {}
    | _ => {}
  }
  ...
}

VD>Ну, почему? Макросами можно решить много чего, в том числе, и эту задачу .

match(t) // t : object
{
 | t is Dictionary['t,'k] => {
    def l = List.['t]();
 }
}

VD>  | x is IDictionary['k,'v] => // Какой будет тип у "x"?
VD>

void m['t](l : List['t]) 
{
   def d = Dictionary.['t, int]();
}

match(t) {
  | _ is Dictionary<,> typed (t1, t2) => {}
}

public class A
{
  private _b : IB;
  this() {
    _b = B();
  }
}

Console.WriteLine("Note, that y="); Console.WriteLine(y); /// это покороче надо записать, но я ваш синтаксис не помню
active match(y)
{
  | Product(2,x) => Console.WriteLine("Here is an integer x, that 2*x=y; such x = "); Console.WriteLine(x);
  | _ => Console.WriteLine("Sorry, there is no such an integer x, that 2*x=y");
}

Sum(a : int, b : int) : option[int]
{
   Some(b - a)
}

active match(5)
{
  | 4 => Console.WriteLine(4);
  | Sum(x,2) => Console.WriteLine(x);
  | _ => Console.WriteLine("You will never see it");
}