An observable value notifies observers of updates in a thread-safe manner. An observable can also be constructed from another observable using the map and filter combinators:

let foo = Observable("Foo")
foo: onChange(|v| -> println("foo = " + v))

let mapped = foo: map(|v| -> v + "!")
mapped: onChange(|v| -> println("mapped = " + v))

foo: set("69")

This yields the following output:

foo = 69
mapped = 69!