Foldable typeclass

[nimiwio]

It would be nice to have a Foldable typeclass similar to Haskell. Then things such as the following would work:

function sum
Foldable f => Integer ::= xs::f Integer { return foldl(\x::Integer y::Integer -> x + y, 0, xs); }

local aSet::Set<Integer> = ...
local aList::[Integer] = ...

local setSum::Integer = sum(aSet);
local listSum::Integer = sum(aList);

In my case, I was hoping for something like flatMap (foldMap in Haskell) to work on Set<a>, but that is specialized to lists only, so I had to write something like:

function foldMapSet
Monoid m => m ::= f::(m ::= a) s::set:Set<a> {
  return flatMap(f, set:toList(s));
}