A prototype implementation of a Bazel persistent worker wrapping GHC.
It uses GHC API for serving compilation requests from Bazel.
Subject of merge into rules_haskell (now it's in the worker branch).
Current implementation does the simplest thing possible: it restarts GHC to serve every request; technically: every request implies a separate call to runGhc. Note that a request here means building one Bazel target, that is, a Haskell binary or Haskell library.
Even simple “full restart” approach brings 10–15% speedup on a certain simple benchmark project. We should do better though. There are several possible avenues for improvement.
There should be a way to profit from the warm startup vs the cold one. One idea I've been pursuing specifically: cache loading package databases. It requires GHC to export some more internals in the API. Subject to a GHC ticket/proposal.
A worker should be able to cache resulting artifacts. Later on, when it recieves a request to build an input with a known digest (hash), it should return the corresponding artifact straight away.
This idea implemented in several other workers (Swift, Kotlin, rules_scala_anex), but not in all (or majority) of them.
cabal new-testshould show off a simple example of building tests/hello.hs.
Because cabal new-build still have rough corners, you have to install stuff manually before calling it. There are two steps.
First, get proto-lens-protoc:
cabal new-install proto-lens-protoc(~/.cabal/bin should be in your path)
Second, get protoc.