Oftentimes, you'll find that some code patterns keep coming up over and over and you'll want to find same way to factor out the major commonalities in logic from the minor specific details that you'd want to just plug in as needed. For example, you might realize that you're writing loops to filter lists based on conditions all over your code; the only difference between the implementation in any of these occurrences of filtering being the element types and the specific condition. But because you want to filter lists of all kinds of types you might not immediately think you could write a single function that could be called wherever filtering is needed. Enter Generics!

Fig 1:

function reduce<A, B>(l: [A], fn: function<|B, A| -> B>, accum: B) -> B {
  for (e in l) {
    accum = fn(accum, e);
  return accum;

The function reduce<A, B>(...) is defined to take a list of elements of some arbitrary (generic) type, A, and an accumulation function that takes in the current accumulated value, of type B, and the current element of that generic type, A. In this example, the particular types A and B are "unconstrained". The only constraint is the typical constraint that the given function's first arg must have the same type as the initial accumulated value, and the second arg must have the same type as the elements of the reduced list.

So, the generic types take on the "concrete" types of the data that happens to be passed into the function's callsite:

Fig 2:

var sum: int = reduce([1, 2, 3, 4], lambda (accum, x) -> accum + x, 0);

var totalChars: int =
    ["Count", "the", "total", "chars", "in", "these", "strings"],
    lambda (accum, s) -> accum + len(s),