"""Abstraction of types which support a binary addition operator `x + y`. For [[numeric types|Numeric]], this is just familiar numeric addition. For [[String]], it is string concatenation. Integer next = current + 1; String helloWorld = "hello" + " " + "world"; A concrete class that implements this interface should be a mathematical _semigroup_. That is, the addition operation should be associative, satisfying: - `(x+y)+z == x+(y+z)` A `Summable` type might be a _monoid_, that is, a semigroup with an additive identity element, usually denoted `0`, but this is not required. For example: - `String` is a monoid with identity element `""`, - `Float` is a monoid with identity element `0.0`, and - `Integer` is a monoid with identity element `0`. For any monoid, the addition operation must satisfy: - `x + 0 == x`""" see (`class String`, `interface Numeric`) by ("Gavin") tagged("Numbers", "Strings") shared interface Summable<Other> of Other given Other satisfies Summable<Other> { "The result of adding the given value to this value. This operation should never perform any kind of mutation upon either the receiving value or the argument value. For any two instances `x` and `y` of a type that implements `Summable`, `x.plus(y)` may be written as: x + y" shared formal Other plus(Other other); }