What this is

Currently this is a toy interpreter for lambda calculus with integers and booleans, and just three operations (addition, equality and if-then-else). Variables are supposed to be lexically scoped, and evaluation order is strict.

How to run it

It actually doesn't run run, like, it's not a program yet. But you can run the tests:

cargo test

What's this supposed to be?

It might eventually become an interpreter that implements this stuff: Free Variable as Effect, in Practice. See notes.md for more details.

What are the issues with it?

Well apart from the fact that it's just a toy, it also has a convoluted architecture in order to make all expressions implement Copy, simplifying ownership (or rather, sidestepping it). The idea is actually pretty cool - I am calling it Sharing to Copy - I just wanted to focus on the lambda calculus rather than make the code too noisy. But I used too many dependencies for it and I would like to cut down a bit. See notes/sharing-to-copy.md for details.

What about the name?

It comes from Having an Effect

This research has been a journey following the tantalizing lead: the founding papers on monads, monad transformers, free monads and extensible-effects were all about extensible interpreters. Along the way we have realized that the expression problem, stable denotations, and extensible interpreters are all different names for the same thing.

In there, the author begins with interpreters that can be extended without modifying previous code, and ends up extending it further (heh) with the idea that variable bindings could be resolved by effect handlers, and well, there it is.