The performance characteristics of immutability

Posted on 2017-10-23

If someone asks “What’s the difference between functional and imperative programming?” it’s tempting to reach for the technical definition which is that “a functional program is composed of functions whereas an object-oriented program is composed of classes”. However, a more fundamental difference between functional languages and their cousins in the object-oriented and procedural spaces is the extensive use of immutability.

The reason I consider this to be a more fundamental difference is because object-oriented programmers with no experience of functional programming (and I include myself from about five years ago in this statement) simply can’t understand how it is possible to write a program in which it’s impossible to change state.
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