In the course of writing my Honours Thesis, I’ve been learning about the lambda calculus, specifically the untyped version. To help me learn about it, I’ve put together a small Javascript app that goes by the original (Church, 1936) definition. I call it Lambda Toy. This app verifies whether or not an expression is a well-formed formula and allows you to carry out renaming and reduction operations.

Church, Alonzo (1936). “An Unsolvable Problem of Elementary Number Theory”. American Journal of Mathematics 58 (58): 345–363.