This is an emulator for the TINY machine, which is a 4-bit Von Neumann architecture.
I first encountered the TINY machine specification through Conor McBride, though I do not know if it was his original work.
The demonstration available in test/Spec.hs is an example of a quine. Running the test -- for example, by stack test -- will initialize a TINY computer's memory with the program 4F5545745CAF3200 and then trace its execution until the computer halts. The output produced will be exactly the original program: 4F5545745CAF3200.
I made use of the Control.Lens library in part because I was curious how it would behave in practice. I find it remarkably useful to inspect and update parts of a multi-layered conceptual structure, with the flexibility to accommodate both field-style ("get me this register") and indexed ("get me this memory location") references into substructures.
While the current list-based input and output streams are sufficient for the demonstration, I would like to rewrite them to use a streaming approach that could either take characters from stdin and write them to stdout in a main executable, or read from and write to lists of characters in a testing context.