I know my mind is decidedly poisoned when I could follow the type definitions perfectly, and they reminded me of certain types I have written myself… ah, TypeScript, what have you done to me…
Also, being able to recognize the Peano S-function on sight (and thus uses of Church-Peano numbers) is a fun part of that wizardry. (Maybe not necessary to remind the usefulness of the Lambda Calculus canon on a site named after one of the most famous combinators.)