skeleton ghc/ghcjs project using nix, stack, cabal, reflex-platform, intero

This is a skeleton project structured to use a bunch of tools in the
ecosystem. Sometimes those tools play nicely together, and sometimes
they don't - this is one configuration in which they do.

Features

• ghc, ghcjs, and all haskell libraries are installed safely and
  deterministically via nix
• ghc-targeted code can be built with either cabal or stack
• ghcjs-targeted code can be built with cabal (but not stack) using ghcjs
• ghcjs-targeted code can be built with stack, using ghc instead of ghcjs
• intero is available for use from editors, for both codebases

Usage

1. install the nix environment
2. in ghc-code, run either stack build or cabal build, as normal
3. in ghcjs-code, to build with stack and ghc just run stack build
4. in ghcjs-code, to build with cabal and ghcjs run cabal configure --ghcjs && cabal build
5. in ghcjs-code, to build with cabal and ghc run rm -r dist; cabal build

To install the nix environment, you have two options

• install it globally, via nix-env -f /path/to/nix-stack-ghcjs-demo/default.nix -i. You can
  uninstall/disable it at any time with nix-env -e ghc-and-ghcjs-env
• open a nix-shell and run everything inside it, via nix-shell -p 'import /path/to/nix-stack-ghcjs-demo'

Details (in no particular order)

• reflex-platform is
  used to provide version pinning. It is itself built on top of nix,
  and it has been carefully put together with versions of various
  packages that work well together (basically, it takes the pain away
  from installing ghcjs).
• I have
  one
  custom commit at
  the top of reflex-platform, which updates intero to a newer version
  (needed by the intero emacs package). If/when reflex-platform
  updates enough to make this no longer necessary, that can be
  removed.
• stack is set up to use the 'system'
  ghc, so it should never try to install anything for you.
• there's only one stack.yaml, which is symlinked inside each of
  ghc-code/ghcjs-code. It needs to be inside those directories, since
  stack tooling looks for stack.yaml to find the project boundary, and
  we want the ghc and ghcjs code to be treated as separate projects.