An exploration of writing a debugger for CEK-style machines.

The machines are all implemented as state machines (see StateMachine.hs).

The machines progress in order:

• A basic CEK machine (CEK.hs)
• A CEK machine with direction control (DCEK.hs)
• A CEK machine with direction control and breakpoints (DCEBK.hs)

You can play around with a state machine by doing something like the following:

> cabal repl
> :m +REPL StateMachine BreakLambda DCEBK Text.Megaparsec Data.Maybe
> repl dcebk (inject (fromJust $ parseMaybe parseExp "(\\x.x)(\\y.y"))
# type "Step"/"Continue"/"Next" etc.

The approach here is very much inspired by [Advice about Debugger Construction] (https://pdfs.semanticscholar.org/7ae9/3a0f1ecac0094051345e518282a333a9df96.pdf), although I think I have managed to find a significantly simpler way of achieving the same thing.