* Hello Windows

  This repository contains the necessary nix-expressions and a ~nixpkgs~
  submodule that should allow us to cross compile a small haskell application
  into a working windows executable.

  [[file:hs-hello.png]]

** Building
   To build the cross compiled ~hs-hello.exe~ with nix, all we need is

   #+BEGIN_SRC bash
   $ nix-build -A hello-world --cores 0 
   #+END_SRC

   This will build the necessary dependencies, and finally build the
   ~hs-hello.exe~, which we can find in ~result/bin/hs-hello.exe~.

** ~config.nix~
   We will need a customized GHC.  Due to the introduction of the
   ~buildPackages~ for the cross compilation capabilities in ~nixpkgs~ this
   turns out to be a bit more complicated than we like.  The core issue here is
   that the stage logic and the ~packageOverrides~ do not play well together.
   Specifically changes made via ~packageOverrides~ do not show up in the
   ~buildPackages~.

** ~hello-world.nix~
   This is our usual haskell package expression.  Just note the additional 

   #+BEGIN_SRC nix
   enableSharedExecutables = false;
   setupHaskellDepends = [ Cabal_HEAD ];
   #+END_SRC

   as we don't have dynamic libraries with our cross compiler (yet).  And need
   to link against a custom ~Cabal~ library.

** ~default.nix~
   In the ~default.nix~ we'll pull everything together.  Set the ~crossSystem~
   to ~mingwW64~ and the ~packageOverlays~ via ~config~.

   In addition we setup a custom ~Cabal~ library, which we want to link against;
   the one that ships with GHC has some defects when cross compiling to windows.

   We use ~callPackage~ on the ~buildPackages~ to ensure that the ~Cabal~
   library is built for the *build* machine.  For the ~hello-world~ package
   (~hello-world.nix~) we use ~pkgs~ which will result in the package to be built
   for the *host* machine.