Consider the example of the generic function:

function filter<T>(l: [T], pred: function<T -> boolean>) -> [T] {...}

If you really squint, you might notice that there's very little information available in the body of the filter<T>(...) function to tell you about the type T. As a result, you're unable to do much with values of such an unconstrained generic type beyond passing the value along to another generic function accepting an unconstrained generic arg, or putting it into some collection defined over the same generic type. This would be very limiting if this was all that could be done with generics.

Enter Contracts! It will take a bit of a buildup, but we should be able to write generic functions that will be able to put constraints on the acceptable types, for example saying something like "this procedure will accept any type, T, for which the function foo(arg1: T, arg2: T) exists."

For example, we should be able to write the following generic function:

requires(Operators<T>)    # <-- What is this `requires(...)`?
function sum<T>(l: [T]) -> T {
    var res = l[0];
    var i = 0;
    while (++i < len(l)) {
        res = Operators::add(res, l[i]); # <-- What is this `Operators::add`?
    return res;

The function above has a new requires(...) clause in the signature which we haven't seen before. This is the mechanism by which a function constrains the set of types that may be passed into this function to only types that definitely have a certain associated procedure implementation existing. The requires(...) clause takes in a list of "Contracts" that must be implemented over the generic type. In this case that contract's definition looks like:

Contract Operators<X> {
    function add(lhs: X, rhs: X) -> X;

This Contract specifies a single function signature that any implementation of this Contract must implement. Other Contracts may specify more than one signature, or even more than one generic type param. There are no restrictions on where the generic Contract param(s) may be used in the procedure signatures, so it may even be included in the return type as shown in the example above.

The only requirement on signatures is that each one must make use of each generic arg type listed in the Contract's signature. This is mandatory as Claro looks up the particular implementations by inspecting the arg types provided at the Contract procedure's call-sites.

Contracts are Not Interfaces

Coming from an Object-Oriented background, you may be tempted to compare Contracts to "Interfaces", but you'll find that while they may be used to a similar effect, they are not the same thing. The intention of an "Interface" is to encode subtyping relationships between types, whereas Claro has absolutely no notion of subtyping. All defined types are strictly independent of one another. Claro asks you to simplify your mental model and simply think of Contracts as a mechanism for encoding a required bit of functionality that needs to be implemented uniquely over values of unrelated, arbitrary (generic) types.