I wonder how Clojure deals with ambiguities when dispatching by arbitrary predicates. Also in this case, the overhead of of generic call will grow linearly with the number of methods, right?
In Python, you could limit generic dispatch to argument type only without losing generality since you could override the `isinstance` operator. For instance, you could define a `const()` type so that `isinstance(x, const(100))` is true if and only if `x == 100`. Then your example could be written in Python as:
foo = generic()
foo.when(const(0))
def foo_0(x): print "passed in zero"
foo.when(const(100))
def foo_100(x): print "passed in 100"
foo.when(const(200))
def foo_200(x): print "passed in 200"
foo.when(int)
def foo_default(x): print "passed in", x