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A lambda calculus interpreter written in typescript with a web interface.

Uses the parsimmon parser combinator library.

I have not verified its correctness beyond a few unit tests, use at your own risk

The lambda calculus is a foundational programming language. Its only values are functions and the only thing that can take place is function application. Other types of values can be represented and manipulated adhoc, for instance the natural numbers can be defined as a successor function and a zero function. Even recursion can be implemented using the famous Y-combinator.

Because of this the lambda calculus is, surprisingly, turing complete. It underpins much of the theory and practice of functional programming.

I decided to write an interpreter because it would be a neat way to get into the basics of parsing and evaluating programs. I also took it as an opportunity to get into the habit of automated testing, enforcing much needed rigour and avoiding a lot of tedium.

Some scattered thoughts:

• Ironically I wrote this in a mostly non-functional way. I'm still learning js/ts so I didn't want to use a functional library yet.
• Writing the parser quickly became a priority as I realised it's also a development tool. Writing object literals for tests was cumbersome.
• The contrast between the way the parser combinator library allowed me to manage complexity, and my originally monolithic evaluator function, emphasised the need for composibility and abstraction at every step. Thinking imperatively is often necessary, but it's best to start out at a higher level than that.
• Typescript's type system is a godsend, but the inference needs improving.
• The fact that I can't use recursion to think recursively about a recursive data structure (the syntax tree) because the language isn't optimised for it is frustrating.
• Drawing diagrams helps to reason
• Approach problems mathematically: break down into simpler cases. Ask questions. The worst thing to do when my mind goes blank is to wait for insights.
• If all else fails, make tea.