Think Perl 6

Chapter 14. Functional Programming in Perl

Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions or declarations instead of statements. In functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function twice with the same argument will produce the same result each time. Eliminating side effects, i.e., changes in state that do not depend on the function inputs, can make it much easier to understand and predict the behavior of a program, which is one of the key motivations for the development of functional programming.

Perl is not a functional language in the sense that it also uses several other programming paradigms that we have seen abundantly throughout this book. It does however offer extensive functional programming features and capabilities, some of which have been introduced in various sections of this book and will be briefly reviewed here before we get further.

Higher-Order Functions

As early as Chapter 3 on functions and subroutines, in “Functions and Subroutines as First-Class Citizens”, we have seen that functions, subroutines, and other code objects are first-class objects or first-class citizens in Perl, which means that they can be passed around as values. A Perl 6 function is a value you can assign ...