This page contains information about the design, implementation, problems and potential solutions for building user-level concurrency primitives in GHC.
All of GHC's concurrency primitives are written in C code and is baked in as a part of the RTS. This precludes extensibility as well as making it difficult to maintain. Ideally, the concurrency libraries will be implemented completely in Haskell code, over a small subset of primitive operations provided by the RTS. This will provide a Haskell programmer the ability to build custom schedulers and concurrency libraries. For an earlier attempt at this problem, please look at Peng Li's paper .
Substrate primitives are the primitives exposed by the RTS, on top of which user-level concurreny libraries are built.