recursion – Recursive definition of the length of a string?

Strings are concatenations of characters. Therefore, any non-empty string $ w $ has a first character $ x $ and can be written as $ w = x w $, or $ w $ is the tail rope. It follows from the definition of the length of a chain that $ | w | = | x w & # 39; = | x | + | w & # 39; = 1 + | w & # 39; $.