This PR has farmed out the type error machinery out to my new package https://github.com/isovector/type-errors. It's much nicer to work with, and a big red diff!
* Remove explicit loopbreakers, enable plugin
* Use uploaded version of `loopbreaker`
* Fix span of macro
* Disable plugin on GHC <8.6
* Add comment about use of plugin
* Separate unrelated conditions
This PR provides a single function withLowerToIO, which runs a desired Sem r effect all the way down to IO, without needing to know the natural transformation beforehand. It does it by running the desired code in a new thread, and shipping all of the unhandled effects back to the main thread. The main thread turns into an event loop for the duration of the withLowerToIO block.
This PR introduces the FCF machinery that will give us more control over writing type-level functions. It defines an IfStuck a b c tyfam that will leave b around if a is stuck, otherwise it will fcf-evaluate c. Everything is polykinded so we can stack these things together to make big logic chains to emit specific variables depending on what exactly is stuck.
This PR adds doctests allowing us to write tests for the custom type errors. Having this stuff reified in the test suite means we can iterate on the implementations and give much better QA.
