Most functional compiler
One-letter variable names abound in IOCCC entries, and for good reason. These tiny pieces of confetti are hard to read, and leave room for more code. Then why not go further and use zero-letter variable names? That is, tacit programming or point-free style.
I had been playing with an algorithm devised by Oleg Kiselyov that effortlessly and efficiently eliminates those pesky variables, leaving behind terms composed from a small set of combinators. No need for lambda lifting or supercombinators.
By adding a handful of lines of mostly parsing code, we get a Haskell compiler, or rather, a compiler that accepts a subset of Haskell sufficiently large to self-host. You might say I wrote a tool for this contest, then ran it on itself to make an entry for it.
And more: https://www.ioccc.org/years.html#2019