Summary

Functions represent another side of the blend of object-oriented and functional features in Scala. They can be defined in a number of ways, including the partial application of methods, function literals, and partial functions. Functions can be defined in any scope. If a function closes over variables available in scope, it is called closure.

Polymorphic functions implement an idea similar to polymorphism in object orientation, but apply that idea for types of parameters and of the result. This is called parametric polymorphism. It is especially useful when defining functions accepting other functions as arguments, so-called higher order functions.

There are two ways to implement recursion and only tail-recursive functions are stack safe in the JVM. For the functions which cannot be made tail-recursive, there is a way to represent the call chain in the heap by encoding it as objects. This approach is called trampolining and it is supported in the standard library.

Functions are first-class values in Scala because they are implemented as anonymous classes extending FunctionN traits. This not only makes it possible to work with functions as with normal variables, but it also allows for the provision of extended function implementations with additional properties, for example, a PartialFunction.