ghc-whole-program-compiler-project VS DocTest

Another thing you can try is the ghc-wpc project which has an interpreter which supports breakpoints, though you may need to hack little a bit to achieve your goals.

If I understand the new GHCJS backend correctly, it diverges from the normal compilation pipeline at STG (see the new StgToJS module subtree). Wouldn't that make it an excellent candidate to use GHC-WPC's machine-readable external STG representation, instead of hooking it directly into the GHC source tree?

Here the goal is to build a high level, easy to understand model for all GHC backend features. Validations is also required. Once we know the semantics of GHC primops and RTS features then it becomes possible to figure out how to compile Haskell programs to GRIN. I started the GHC-WPC project for this reason. GHC-WPC exports the STG intermediate representation for the whole Haskell program, and I wrote an STG interpreter from scratch in Haskell that can run any Haskell program. (i.e. GHC itself) The STG interpreter is the high level model for the GHC primop and RTS semantics. It implements all these in pure Haskell, it does not depend on GHC RTS at all.

I can easily debug any Haskell program with the external STG interpreter. https://www.youtube.com/watch?v=DkDUEd3pUyM https://github.com/grin-compiler/ghc-whole-program-compiler-project

Hello, I'm using the external STG interpreter to observe the runtime behaviour of Haskell programs. Lately I've added an initial call-graph construction feature that I plan to refine further. https://twitter.com/csaba_hruska/status/1417486380536582151 Is there anyone who has dynamic analysis related research ambitions and wants to study Haskell program runtime behaviour in detail? If so then it would be great to talk.

I haven't gotten my hands dirty yet, but really excited hearing GHC-WPC is going on!

Why didn't you mention GHC-WPC? It is also a backend sample. It exports enough information (STG + linker opts + c bits) to interpret the program or to generate a binary executable via the regular GHC codegen system. https://github.com/grin-compiler/ghc-whole-program-compiler-project

You could use the GHC codegen and RTS via the external STG IR. https://github.com/grin-compiler/ghc-whole-program-compiler-project

Remarks: 1. Strict functional languages can be expressed in STG without overhead, because STG has explicit liftedness control. In a strict language every data is unlifted or unboxed. 2. Supporting all GHC primops is not unrealistic. See the primop implementation in the external STG interpreter source code. Here is the implementation of the threading primops.

doctest for testing examples in the documentation. Docs are much clearer when they contain usage examples and doctest helps them keep up to date.

Try to restrict your types even on sub functions (inside where), use testing, break down your code to the most atomic parts, using ghci to debug each part once at a time, and because Haskell doesn't let you reuse variables, or mutate state, it's a lot easier to rationalize evaluation order (which makes it a lot easier to debug without step debuggers compared to languages like python).

Some may argue that the tests are a form of documentation, and that's true, but they still don't capture the why. And tests are almost always separated from the actual code, which makes it more complicated to look at the code and the tests at the same time and understand them together. I've used doctest with Haskell which is a solution to this. Doctest is nice at first but in my experience it wasn't reasonable to do non-trivial tests in doctest, and plenty of tests are non-trivial. So then you're left with the decision to use doctest where you can, and another test solution for non-trivial tests, or just do all tests using the other testing solution. We chose simplicity and got rid of doctest.