Using the following expression:
```
with import ./. { nixpkgs = ../nixpkgs; nixpkgsArgs = { crossSystem = { config = "js-unknown-ghcjs"; }; }; };
let Cabal = buildPackages.haskell-nix.hackage-package {
    name = "Cabal"; version = "2.4.1.0";
    modules = [
        { packages.Cabal.patches = [ ./Cabal-install-folder.diff ]; }
    ];
}; in
(haskell-nix.hackage-package {
    name = "hello"; version = "1.0.0.2";
    modules = [
    	    ({config, ... }:{ packages.hello.package.setup-depends = [ Cabal ]; })
    ];}).components.exes.hello
```
in a `test.nix` file. And running
```
nix build -f ./test.nix
```
on it, will produce
```
./result
├── bin
│   └── hello.jsexe
│       ├── all.js
│       ├── all.js.externs
│       ├── index.html
│       ├── lib.js
│       ├── manifest.webapp
│       ├── out.frefs.js
│       ├── out.frefs.json
│       ├── out.js
│       ├── out.stats
│       ├── rts.js
│       └── runmain.js
└── share
    └── doc
        └── x86_64-linux-ghc-8.6.5
            └── hello-1.0.0.2
                └── LICENSE

6 directories, 12 files
```

assuming that `nixpkgs` points to NixOS/nixpkgs#67359.

This moves the customizations we did in iohk-nix
into haskell.nix via overlays and config.

Add bootPkgs logic

this moves the nuking of bootPkgs where it belongs. This should eventually still be
removed and replaced by a proper solution, that doesn't require the nuking of
bootPkgs.

Allow us to bootstrap binary ghcs and a cabal-install

With this we can do the following:

```
$ nix repl compiler/old-ghc-nix
nix-repl> :b (let pkgs = import <nixpkgs> {}; in with import ./. {}; import ./compiler/bootstrap/cabal-install.nix { inherit (pkgs) fetchurl stdenv zlib; inherit hackage
; ghc = ghc844; src = pkgs.fetchurl { url = "https://github.com/haskell/cabal/archive/Cabal-v3.0.0.0-rc3.tar.gz"; sha256 = "1zl2mgg8307ykq3v8nmafc6zdhhj1cw7w8ffpap16dsm6
5lbnx33"; }; })
```

which wile it may look daunting, will allow us to bootstrap a
cabal-install with a ghc. From that point onwards, we should be able to
build any hackage package via haskell.nix.

Pass through cabal-install version

Better threading of arguments.

Add bootstrap overlay

Allow alex + happy to be built

This still has a wart: we need nix-tools, and for that we use the ghc865 from
nixpkgs. Ideally we'd build nix-tools against a specific ghc, but then
we'd need a build expression for that.

Make ghcjs work

Building something like this:
```
 nix build '(with import ./. { nixpkgs = ../nixpkgs; nixpkgsArgs = { crossSystem = { config = "js-unknown-ghcjs"; }; }; }; (haskell-nix.hackage-package { name = "hello"; version = "1.0.0.2"; })).components.exes.hello'
```
will now work. Assuming `nixpkgs` has been appropriately patched to support the `js-unknown-ghcjs` triple.

Also: this will need an additional `Cabal` patch, to make `Cabal` understand what it needs to do with: `dist/build/hello/hello: copyFile: does not exist (No such file or directory)`

It needs to learn that `dist/build/hello/hello.jsexe` is what it wants to copy and that that is a directory.  Luckily we do have some code in Cabal already that does this for `.exe` on windows.

Build `js-unknown-ghcjs` packages with haskell.nix

Using the following expression:
```
with import ./. { nixpkgs = ../nixpkgs; nixpkgsArgs = { crossSystem = { config = "js-unknown-ghcjs"; }; }; };
let Cabal = buildPackages.haskell-nix.hackage-package {
    name = "Cabal"; version = "2.4.1.0";
    modules = [
        { packages.Cabal.patches = [ ./Cabal-install-folder.diff ]; }
    ];
}; in
(haskell-nix.hackage-package {
    name = "hello"; version = "1.0.0.2";
    modules = [
    	    ({config, ... }:{ packages.hello.package.setup-depends = [ Cabal ]; })
    ];}).components.exes.hello
```
in a `test.nix` file. And running
```
nix build -f ./test.nix
```
on it, will produce
```
./result
├── bin
│   └── hello.jsexe
│       ├── all.js
│       ├── all.js.externs
│       ├── index.html
│       ├── lib.js
│       ├── manifest.webapp
│       ├── out.frefs.js
│       ├── out.frefs.json
│       ├── out.js
│       ├── out.stats
│       ├── rts.js
│       └── runmain.js
└── share
    └── doc
        └── x86_64-linux-ghc-8.6.5
            └── hello-1.0.0.2
                └── LICENSE

6 directories, 12 files
```

assuming that `nixpkgs` points to NixOS/nixpkgs#67359.

This moves the customizations we did in iohk-nix
into haskell.nix via overlays and config.

Add bootPkgs logic

this moves the nuking of bootPkgs where it belongs. This should eventually still be
removed and replaced by a proper solution, that doesn't require the nuking of
bootPkgs.

Allow us to bootstrap binary ghcs and a cabal-install

With this we can do the following:

```
$ nix repl compiler/old-ghc-nix
nix-repl> :b (let pkgs = import <nixpkgs> {}; in with import ./. {}; import ./compiler/bootstrap/cabal-install.nix { inherit (pkgs) fetchurl stdenv zlib; inherit hackage
; ghc = ghc844; src = pkgs.fetchurl { url = "https://github.com/haskell/cabal/archive/Cabal-v3.0.0.0-rc3.tar.gz"; sha256 = "1zl2mgg8307ykq3v8nmafc6zdhhj1cw7w8ffpap16dsm6
5lbnx33"; }; })
```

which wile it may look daunting, will allow us to bootstrap a
cabal-install with a ghc. From that point onwards, we should be able to
build any hackage package via haskell.nix.

Pass through cabal-install version

Better threading of arguments.

Add bootstrap overlay

Allow alex + happy to be built

This still has a wart: we need nix-tools, and for that we use the ghc865 from
nixpkgs. Ideally we'd build nix-tools against a specific ghc, but then
we'd need a build expression for that.

Make ghcjs work

Building something like this:
```
 nix build '(with import ./. { nixpkgs = ../nixpkgs; nixpkgsArgs = { crossSystem = { config = "js-unknown-ghcjs"; }; }; }; (haskell-nix.hackage-package { name = "hello"; version = "1.0.0.2"; })).components.exes.hello'
```
will now work. Assuming `nixpkgs` has been appropriately patched to support the `js-unknown-ghcjs` triple.

Also: this will need an additional `Cabal` patch, to make `Cabal` understand what it needs to do with: `dist/build/hello/hello: copyFile: does not exist (No such file or directory)`

It needs to learn that `dist/build/hello/hello.jsexe` is what it wants to copy and that that is a directory.  Luckily we do have some code in Cabal already that does this for `.exe` on windows.

Build `js-unknown-ghcjs` packages with haskell.nix

Using the following expression:
```
with import ./. { nixpkgs = ../nixpkgs; nixpkgsArgs = { crossSystem = { config = "js-unknown-ghcjs"; }; }; };
let Cabal = buildPackages.haskell-nix.hackage-package {
    name = "Cabal"; version = "2.4.1.0";
    modules = [
        { packages.Cabal.patches = [ ./Cabal-install-folder.diff ]; }
    ];
}; in
(haskell-nix.hackage-package {
    name = "hello"; version = "1.0.0.2";
    modules = [
    	    ({config, ... }:{ packages.hello.package.setup-depends = [ Cabal ]; })
    ];}).components.exes.hello
```
in a `test.nix` file. And running
```
nix build -f ./test.nix
```
on it, will produce
```
./result
├── bin
│   └── hello.jsexe
│       ├── all.js
│       ├── all.js.externs
│       ├── index.html
│       ├── lib.js
│       ├── manifest.webapp
│       ├── out.frefs.js
│       ├── out.frefs.json
│       ├── out.js
│       ├── out.stats
│       ├── rts.js
│       └── runmain.js
└── share
    └── doc
        └── x86_64-linux-ghc-8.6.5
            └── hello-1.0.0.2
                └── LICENSE

6 directories, 12 files
```

assuming that `nixpkgs` points to NixOS/nixpkgs#67359.