# User's Guide to the Erlang Infrastructure

## Build Tools

By default Rebar3 wants to manage it's own dependencies. In the normal non-Nix, this is perfectly acceptable. In the Nix world it is not. To support this we have created two versions of rebar3, `rebar3` and `rebar3-open`. The `rebar3` version has been patched to remove the ability to download anything from it. If you are not running it a nix-shell or a nix-build then its probably not going to work for you. `rebar3-open` is the normal, un-modified rebar3. It should work exactly as would any other version of rebar3. Any Erlang package should rely on `rebar3` and thats really what you should be using too.

## How to install Erlang packages

Erlang packages are not registered in the top level simply because they are not relevant to the vast majority of Nix users. They are installable using the `erlangPackages` attribute set.

You can list the avialable packages in the `erlangPackages` with the following command:

    $ nix-env -f "<nixpkgs>" -qaP -A erlangPackages
    erlangPackages.esqlite    esqlite-0.2.1
    erlangPackages.goldrush   goldrush-0.1.7
    erlangPackages.ibrowse    ibrowse-4.2.2
    erlangPackages.jiffy      jiffy-0.14.5
    erlangPackages.lager      lager-3.0.2
    erlangPackages.meck       meck-0.8.3
    erlangPackages.rebar3-pc  pc-1.1.0

To install any of those packages into your profile, refer to them by their attribute path (first column):

    $ nix-env -f "<nixpkgs>" -iA erlangPackages.ibrowse

The attribute path of any Erlang packages corresponds to the name of that particular package in Hex or its OTP Application/Release name.

## Packaging Erlang Applications

### Rebar3 Packages

There is a Nix functional called `buildRebar3`. We use this function to make a derivation that understands how to build the rebar3 project. For example, the epression we use to build the hex2nix project follows.

    {stdenv, fetchFromGitHub, buildRebar3, ibrowse, jsx, erlware_commons }:

    buildRebar3 rec {
        name = "hex2nix";
        version = "0.0.1";

        src = fetchFromGitHub {
            owner = "ericbmerritt";
            repo = "hex2nix";
            rev = "${version}";
            sha256 = "1w7xjidz1l5yjmhlplfx7kphmnpvqm67w99hd2m7kdixwdxq0zqg";
        };

        erlangDeps = [ ibrowse jsx erlware_commons ];
    }

The only visible difference between this derivation and something like `stdenv.mkDerivation` is that we have added `erlangDeps` to the derivation. If you add your Erlang dependencies here they will be correctly handled by the system.

If your package needs to compile native code via Rebar's port compilation mechenism. You should add `compilePort = true;` to the derivation.

### Hex Packages

Hex packages are based on Rebar packages. In fact, at the moment we can only compile Hex packages that are buildable with Rebar3. Packages that use Mix and other build systems are not supported. That being said, we know a lot more about Hex and can do more for you.

    { buildHex }:
      buildHex {
        name = "esqlite";
        version = "0.2.1";
        sha256 = "1296fn1lz4lz4zqzn4dwc3flgkh0i6n4sydg501faabfbv8d3wkr";
        compilePort = true;
    }

For Hex packages you need to provide the name, the version, and the Sha 256 digest of the package and use `buildHex` to build it. Obviously, the package needs to have already been published to Hex.

## How to develop

### Accessing an Environment

Often, all you want to do is be able to access a valid environment that contains a specific package and its dependencies. we can do that with the `env` part of a derivation. For example, lets say we want to access an erlang repl with ibrowse loaded up. We could do the following.

          ~/w/nixpkgs ❯❯❯ nix-shell -A erlangPackages.ibrowse.env --run "erl"
          Erlang/OTP 18 [erts-7.0] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]

          Eshell V7.0  (abort with ^G)
          1> m(ibrowse).
          Module: ibrowse
          MD5: 3b3e0137d0cbb28070146978a3392945
          Compiled: January 10 2016, 23:34
          Object file: /nix/store/g1rlf65rdgjs4abbyj4grp37ry7ywivj-ibrowse-4.2.2/lib/erlang/lib/ibrowse-4.2.2/ebin/ibrowse.beam
          Compiler options:  [{outdir,"/tmp/nix-build-ibrowse-4.2.2.drv-0/hex-source-ibrowse-4.2.2/_build/default/lib/ibrowse/ebin"},
          debug_info,debug_info,nowarn_shadow_vars,
          warn_unused_import,warn_unused_vars,warnings_as_errors,
          {i,"/tmp/nix-build-ibrowse-4.2.2.drv-0/hex-source-ibrowse-4.2.2/_build/default/lib/ibrowse/include"}]
          Exports:
          add_config/1                  send_req_direct/7
          all_trace_off/0               set_dest/3
          code_change/3                 set_max_attempts/3
          get_config_value/1            set_max_pipeline_size/3
          get_config_value/2            set_max_sessions/3
          get_metrics/0                 show_dest_status/0
          get_metrics/2                 show_dest_status/1
          handle_call/3                 show_dest_status/2
          handle_cast/2                 spawn_link_worker_process/1
          handle_info/2                 spawn_link_worker_process/2
          init/1                        spawn_worker_process/1
          module_info/0                 spawn_worker_process/2
          module_info/1                 start/0
          rescan_config/0               start_link/0
          rescan_config/1               stop/0
          send_req/3                    stop_worker_process/1
          send_req/4                    stream_close/1
          send_req/5                    stream_next/1
          send_req/6                    terminate/2
          send_req_direct/4             trace_off/0
          send_req_direct/5             trace_off/2
          send_req_direct/6             trace_on/0
          trace_on/2
          ok
          2>

Notice the `-A erlangPackages.ibrowse.env`.That is the key to this functionality.

### Creating a Shell

Getting access to an environment often isn't enough to do real development. Many times we need to create a `shell.nix` file and do our development inside of the environment specified by that file. This file looks a lot like the packageing described above. The main difference is that `src` points to project root and we call the package directly.

    { pkgs ? import "<nixpkgs"> {} }:

    with pkgs;

    let

      f = { buildHex, ibrowse, jsx, erlware_commons }:
          buildHex {
            name = "hex2nix";
            version = "0.1.0";
            src = ./.;
            erlangDeps = [ ibrowse jsx erlware_commons ];
          };
      drv = erlangPackages.callPackage f {};

    in
     drv

#### Building in a shell

Unfortunatly for us users of Nix, Rebar isn't very cooperative with us from the standpoint of building a hermetic environment. When building the rebar3 support we had to do some sneaky things to get it not to go out and pull packages on its own. Also unfortunately, you have to do some of the same things when building a project inside of a Nix shell.

1. Run `rebar3-nix-bootstrap` every time dependencies change

2. Set Home to the current directory.

If you do these two things then Rebar will be happy with you. I codify these into a makefile. Forunately, rebar3-nix-bootstrap is idempotent and fairly quick. so you can run it as often as you like.

    # =============================================================================
    # Rules
    # =============================================================================
    .PHONY= all test clean repl shell build test analyze bootstrap

    all: test

    clean:
            rm -rf _build
            rm -rf .cache

    repl:
            nix-shell --run "erl"

    shell:
            nix-shell --run "bash"

    bootstrap:
            nix-shell --pure --run "rebar3-nix-bootstrap"

    build: bootstrap
            nix-shell --pure --run "HOME=$(CURDIR) rebar3 compile"

    analyze: bootstrap
            nix-shell --pure --run "HOME=$(CURDIR) rebar3 do compile,dialyzer"

    test: bootstrap
            nix-shell --pure --run "HOME=$(CURDIR) rebar3 do compile,dialyzer,eunit"

If you add the `shell.nix` as described and user rebar as follows things should simply work.

## Generating Packages from Hex with Hex2Nix

Updating the Hex packages requires the use of the `hex2nix` tool. Given the path to the Erlang modules (usually `pkgs/development/erlang-modules`). It will happily dump a file called `hex-packages.nix`. That file will contain all the packages that use a recognized build system in Hex. However, it can't know whether or not all those packages are buildable.

To make life easier for our users, it makes good sense to go ahead and attempt to build all those packages and remove the ones that don't build. To do that, simply run the command (in the root of your `nixpkgs` repository). that follows.

    $ nix-build -A erlangPackages

That will build every package in `erlangPackages`. Then you can go through and manually remove the ones that fail. Hopefully, someone will improve `hex2nix` in the future to automate that.