How is a Rope different from a String?

A comparison of two data structures

A String is a common data type used in programming that every programmer should be familiar with. Unfortunately the benefits of Rope are lesser known.

All Strings are Rope

To start explaining the Rope data structure, let’s look at its type signature.

type Rope = String | (Rope,Rope)

From this we can see that all String are Rope. This is the simplest case, and demonstrates one of the benefits of Rope: all the good features of Strings are directly features of Rope.

Rope is a Braided String

The second case of Rope is simplest to understand in terms of concatenation of Strings. If you have two Strings, then putting them together makes a Rope. The trick is that no copying of data takes place. Instead, the Rope just stores a pointer to each String. This optimization changes the time complexity of concatenation from O(n) linear time to O(1) constant time.

All Rope can be Converted to a String

If it becomes necessary to have a flat String of data then it is trivial to move all Rope data into a single String. This conversion can provide compatibility with legacy code. There really are very few downsides to using Rope vs String.