You wouldn&#x27;t. I think saying you used category theory to design something just sounds cool and is basically a shibboleth. In fact, any time you compose functions you&#x27;re using category theory. I think it sounds a little pretentious to be honest. No one says they used boolean logic when they wrote some code so saying you used category theory or type theory sounds equally weird.<p>I work with a guy that has a PhD in some heavy duty categorical machinery and I haven&#x27;t heard him once say he used category theory to design some piece of code.