Usage of ghc.BUILD to import threaded-rts should be documented

[guibou]

The tutorial only shows how to import ghc from nixpkgs using nixpkgs_package or a custom rule such as:

new_local_repository(
  name = "my_ghc",
  path = "/usr/local", # Change path accordingly.
  build_file_content = """
package(default_visibility = ["//visibility:public"])
filegroup(
    name = "bin",
    srcs = glob(["bin/*"]),
)
"""
)

Unfortunately this is incomplete and does not import the threaded-rts library. PR #436 shows that it is not trivial and introduce the following lines inside a file ghc.BUILD (see https://github.com/tweag/rules_haskell/blob/fcfa800a59b6d2323ce3b47d76e15b68c289356e/haskell/ghc.BUILD):

cc_library(
  name = "threaded-rts",
  srcs = glob(
      ["lib/ghc-*/rts/libHSrts_thr-ghc*." + ext for ext in ["so", "dylib"]]
      # dependency of `libHSrts_thr_ghc*`
      # globbing on the `so` version to stay working when they update
      + ["lib/ghc-*/rts/libffi.so.*"],
  ),
  hdrs = glob(["lib/ghc-*/include/**/*.h"]),
  strip_include_prefix = glob(["lib/ghc-*/include"], exclude_directories=0)[0],
)

Most projects using the threaded-rts needs te copy/paste this boilerplate. This is not documented and error prone. We need to find a way te expose this file and avoid this boilerplate, or document.

[lunaris]

If I ever pick up #387 again, this might be something we work into that (providing haskell_nixpkgs_toolchain, haskell_local_toolchain etc.) I've currently parked that to work on (read: pester/file bugs) against #442; the two together will I think make rules_haskell something very user friendly (and possibly remove the "need" for Hazel, in most cases)

[mboes]

With the introduction of haskell_register_ghc_bindists() and haskell_register_ghc_nixpkgs(), this is now resolved. Both macros use ghc.BUILD, which in turn exposes the threaded RTS. The user doesn't need to do anything here.