tldr; reimplement standard library functions in your favorite language without loops.
Background
For a few years after college I spent a lot of free time doing projects in Standard ML and Scheme. As a result I got really comfortable doing recursion. The two big reasons for this are 1) neither Standard ML or Scheme have loops and 2) they both have very small standard libraries. (Ok, they have loops. They’re just so limited as to be useless.)
I ended up building a standard library for Standard ML including string functions (contains, indexOf, count, replace, etc.), an HTTP server and client, a hash table, a binary search tree, parts of a Standard ML parser, and so on.
All of this without loops…
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