ghc-shake

This is a reimplementation of ghc --make using Shake.  It's entirely one
process, so unlike out-of-process Shake, you won't see any slowdown
compared to --make if you compile with only one core.

# What's the benefit?

    1. You get faster rebuild checking, because Shake doesn't
    recompute the entire dependency graph (ghc --make must
    preprocess and parse all source code every time it loads.)

    2. You can keep going even when one file fails to compile,
    making sure that anything can be built, is built.

    3. You get all the nice Shake features, e.g. unchanging
    rebuilds and profiling.  The profiling is really
    handy!

# How do I use it?

You'll need these two (somewhat hard to get) items:

    - GHC 8.0

    - A customized version of Shake: https://github.com/ezyang/shake

    - Appropriately patched copies of all of Shake's dependencies
      to work with GHC 8.0.  If you wait, these will probably work
      out; but at the time of writing I needed to pass
      --allow-newer=base,transformers,process to cabal to get
      all of the dependencies to install.

One way to to get everything installed (assuming GHC 8.0 is in your
path) is to run:

    git clone https://github.com/ezyang/shake
    (cd shake && cabal install --allow-newer=base,transformers,process)
    git clone https://github.com/ezyang/ghc-shake
    (cd ghc-shake && cabal install --allow-newer=base,transformers,process)

This will enable usage of `ghc --frontend GhcShake`.  We have a sample
script 'ghc-shake' in our source directory which you can pass to Cabal
as a `--with-ghc` argument, which will override the meaning of `--make`
(however, you'll need to edit the `-plugin-package-db` arguments to
accurately reflect your installation.  If you're not building ghc-shake
in place, deleting them should be good enough.)

# What doesn't work

    1. Re-linking detection across packages.  See
        https://ghc.haskell.org/trac/ghc/ticket/10161

    2. Profiling.  See
        https://ghc.haskell.org/trac/ghc/ticket/11293

    3. Linking things that are not executables

    4. Recompilation after package database changes.  (We
    do correctly pick up changes to external interface
    files, however.)

    5. ghc-shake is not really lib-ified properly (it
    should be namespaced and given explicit exports).
    I'll do this eventually.

    6. hs-boot loops don't work; we don't properly
    invalidate the EPS when we successfully compile
    the replacement for an hs file.

If you need something, shout and I'll try to implement it.