In stork you can't formally define interfaces, but they exist through conventions. They exist as informal contract for a function, that is required to have certain parameters and return type.
Predicate is a function with 1 parameter that returns boolean. It tells if that argument matches predicate.
Predicates can be defined as standalone function.
divisibleBy(divisor)(number) {
equal(0)(modulo(divisor)(number))
}
They can be built by currying binary integer operators: equal(3), atLeast(5).
They are used in stream functions filter, while, contains.
Equal is a function with 2 parameters that returns boolean. It tells if 2 arguments are equal. Objects in stork don't have built-in equality function, so some higher-order function require providing explicit function. For example function that tests equality of streams lang.stream.equal requires providing function for testing equality of elements in those streams.
Comparator is a function with 2 parameters that returns boolean. It tells if they are in ascending order. It is used as parameter in sorting algorithms. Order of sorting can be reversed using lang.function.flip on comparator.