# Beam Languages (Erlang & Elixir)

## Introduction

In this document and related Nix expressions we use the term Beam to describe the environment. Beam is the name of the Erlang Virtial Machine and, as far as we know, from a packaging perspective all languages that run on Beam are interchangable. The things that do change, like the build system, are transperant to the users of the package. So we make no distinction.

## Build Tools

### Rebar3

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.

### Mix & Erlang.mk

Both Mix and Erlang.mk work exactly as you would expect. There is a bootstrap process that needs to be run for both of them. However, that is supported by the `buildMix` and `buildErlangMk` derivations.

## How to install Beam packages

Beam 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 `beamPackages` attribute set.

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

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

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

## Packaging Beam Applications

### 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";
                };

              beamDeps = [ 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 Beam 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.

#### Erlang.mk Packages

Erlang.mk functions almost identically to Rebar. The only real difference is that `buildErlangMk` is called instead of `buildRebar3`

            { buildErlangMk, fetchHex,  cowlib, ranch }:
              buildErlangMk {
                name = "cowboy";
                version = "1.0.4";
                src = fetchHex {
                  pkg = "cowboy";
                  version = "1.0.4";
                  sha256 =
                    "6a0edee96885fae3a8dd0ac1f333538a42e807db638a9453064ccfdaa6b9fdac";
                };
                beamDeps  = [ cowlib ranch ];

                meta = {
                  description = ''Small, fast, modular HTTP server written in
                                Erlang.'';
                  license = stdenv.lib.licenses.isc;
                  homepage = "https://github.com/ninenines/cowboy";
                };
            }

#### Mix Packages

Mix functions almost identically to Rebar. The only real difference is that `buildMix` is called instead of `buildRebar3`

            { buildMix, fetchHex, plug, absinthe }:
            buildMix {
              name = "absinthe_plug";
              version = "1.0.0";
              src = fetchHex {
                pkg = "absinthe_plug";
                version = "1.0.0";
                sha256 =
                  "08459823fe1fd4f0325a8bf0c937a4520583a5a26d73b193040ab30a1dfc0b33";
              };
              beamDeps  = [ plug absinthe];

              meta = {
                description = ''A plug for Absinthe, an experimental GraphQL
                                toolkit'';
                license = stdenv.lib.licenses.bsd3;
                homepage = "https://github.com/CargoSense/absinthe_plug";
             };
           }

## 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 beamPackages.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 beamPackages.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 = { buildRebar3, ibrowse, jsx, erlware_commons }:
          buildRebar3 {
            name = "hex2nix";
            version = "0.1.0";
            src = ./.;
            erlangDeps = [ ibrowse jsx erlware_commons ];
          };
      drv = beamPackages.callPackage f {};

    in
     drv

#### Building in a shell

We can leveral the support of the Derivation, regardless of which build Derivation is called by calling the commands themselv.s

    # =============================================================================
    # Variables
    # =============================================================================

    NIX_TEMPLATES := "$(CURDIR)/nix-templates"

    TARGET := "$(PREFIX)"

    PROJECT_NAME := thorndyke

    NIXPKGS=../nixpkgs
    NIX_PATH=nixpkgs=$(NIXPKGS)
    NIX_SHELL=nix-shell -I "$(NIX_PATH)" --pure
    # =============================================================================
    # Rules
    # =============================================================================
    .PHONY= all test clean repl shell build test analyze configure install \
            test-nix-install publish plt analyze

    all: build

    guard-%:
            @ if [ "${${*}}" == "" ]; then \
                    echo "Environment variable $* not set"; \
                    exit 1; \
            fi

    clean:
            rm -rf _build
            rm -rf .cache

    repl:
            $(NIX_SHELL) --run "iex -pa './_build/prod/lib/*/ebin'"

    shell:
            $(NIX_SHELL)

    configure:
            $(NIX_SHELL) --command 'eval "$$configurePhase"'

    build: configure
            $(NIX_SHELL) --command 'eval "$$buildPhase"'

    install:
            $(NIX_SHELL) --command 'eval "$$installPhase"'

    test:
            $(NIX_SHELL) --command 'mix test --no-start --no-deps-check'

    plt:
            $(NIX_SHELL) --run "mix dialyzer.plt --no-deps-check"

    analyze: build plt
            $(NIX_SHELL) --run "mix dialyzer --no-compile"

If you add the `shell.nix` as described and user rebar as follows things should simply work. Aside from the `test`, `plt`, and `analyze` the talks work just fine for all of the build Derivations.

## 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 beamPackages

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