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De Bruijn also invented De Bruijn indices for handling bound variables in lambda calculus without having to deal with renaming. De Bruijn indices are used to implement practically every modern functional programming language. They are also quite beautiful. http://en.wikipedia.org/wiki/De_Bruijn_index

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I learned about why De Bruijn indices are awesome just a couple of days after his death--very unfortunate timing. I only learned about it after I started appreciating them...

(Basically, they are neat because they let you represent a lambda as just a body and an environment as just a list of values, elegantly avoiding the pitfalls of using named variables.)

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