elixir

Decorate functions using macros in Elixir

After I decided to make public a telegram bot to monitor bus time in Dublin (@dublin_bus_bot). Before the release I became curious to see how many people will use it (spoiler: just an handful) and I thought that would be a good idea to track the use on google analytics.

Overview

Google analytics provide a measurement protocol that can be used to track things that are different from websites (mobile apps, IOT). At the moment no elixir client exists for this protocol (and it would not be anything more than an api wrapper). My plan is to make call to the Google Analytics TK endpoint with HTTPOison but I’d prefer to not have to call the tracking function for every single bot command.

One of the feature that I prefer of the elixir are macros, macros allow to generate code at compile time. I decided to define a macro that looking like a function definition would define a function with the same body and with an additional call to the track function. I decided this approach because seems more idiomatics than using the decorator syntax typical of other languages (@decorator at least in python and javascript).

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defmetered sample_function(arg1, arg2) do
IO.inspect([arg1, arg2])
end
# would generate something similar to
def sample_function(arg1, arg2) do
track(:sample_function, [arg1: arg1, arg2: arg2])
IO.inspect([arg1, arg2])
end

Implementation

I implemented this approach in meter to use in the telegram bot I wrote.

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@doc """
Replace a function definition, automatically tracking every call to the function
on google analytics. It also track exception with the function track_error.
This macro intended use is with a set of uniform functions that can be concettualy
mapped to pageviews (eg: messaging bot commands).
Example:
defmetered function(arg1, arg2), do: IO.inspect({arg1,arg2})
function(1,2)
will call track with this parameters
track(:function, [arg1: 1, arg2: 2])
Additional parameters will be loaded from the configurationd
"""
# A macro definition can use pattern matching to destructure the arguments
defmacro defmetered({function,_,args} = fundef, [do: body]) do
# arguments are defined in 3 elements tuples
# this extract the arguments names in a list
names = Enum.map(args, &elem(&1, 0))
# meter will contain the body of the function that will be defined by the macro
metered = quote do
# quote and unquote allow to switch context,
# simplyfing a lot quoted code will run when the function is called
# unquoted code run at compile time (when the macro is called)
values = unquote(
args
|> Enum.map(fn arg -> quote do
# allow to access a value at runtime knowing the name
# elixir macros are hygienic so it's necessary to mark it
# explicitly
var!(unquote(arg))
end
end)
)
# Match argument names with their own values at call time
map = Enum.zip(unquote(names), values)
# wrap the original function call with a try to track errors too
try do
to_return = unquote(body)
track(unquote(function), map)
to_return
rescue
e ->
track_error(unquote(function), map, e)
raise e
end
end
# define a function with the same name and arguments and with the augmented body
quote do
def(unquote(fundef),unquote([do: metered]))
end
end

Conclusions

Elixir macros are a powerful tool to abstract away some functionality or to write DSLs. They require a bit of time to wrap head around, in particular with the context swith, but it totally worth the hassle if you can reduce the clutter in your code base.

The 3 E: Elixir, Exrm, and Environment variables

Intro

I’m building a bot for Telegram, once make a build with exrm I found myself some problem configuring the telegram api key using environment variables. I decided to share what I found because my google foo was not helpful at all.

TL;DR

To configure en elixir application built with exrm use conform and load the environment variable in the trasforms section of the conform schema.

config.exs

config.exs is where the configuration of elixir project are added. The file is interpreted when a project is ran with ies -S mix or mix run.

The values are retrieved during the lifetime of the application with the functions(get_env/3, fetch_env/2, fetch_env!/2) available in the module Application. To include values from the environment where the project is running System.get_env/1 is used.

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use Mix.Config
config :plug,
key1: "value1",
key2: System.get_env("KEY")
import_config "#{Mix.env}.exs"

exrm

Surprisingly (if you did not have an erlang background as me) when you make a release of your project with exrm the config.exs file is executed at build time and the environment variables are crystallized in the build output.

The output from exrm contains a file named sys.config that is the output of executing the config.exs file and is defined as erlang terms. Once released editing this file is the only way to dynamically configure the application once built.

conform

conform is a library from the same author of exrm and is been made to ease the configuration of elixir application. The library validate a property like file (configuration/your_app.conf) against a configuration schema (configuration/your_app.schema.exs) and generate the sys.config file. The schema file contains descriptions, defaults, and types of the parameters. A property file is a lot easier and common to configure than a file containing erlang terms, additionaly conform add flexibility and more control over configurations.

A couple of task are made available to transition to a conform based configuration.

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#generate the schema from the actual configuration
mix conform.new
#generate a .conf file from the configuration
mix conform.configure

The sys.config is generated at the start of the application, and using the plugin exrm_conform at the start of a packaged application. This behaviour allow to load configuration parameter from environment variables defined when the application is started.

After running mix conform.new you will find a yourapp.schema.exs in your conf folder, this file has 3 main sections: mapping, transforms, and validators. The mapping section is where the parameters are defined and where you can set up defaults, descriptions, and type. The transform section allow to add transformation function to change or derive a configured parameter. In the end the validators sections allow to reject invalid configuration errors and stop the application.

At first I tried to add the reading from the environment variable to the default of the parameters, but this lend to an uncommon situation where a static parameter (the one in yourapp.conf) will override a parameter derived from an environment variable.

Eventually I found that adding a function to transforms is probably a better way to do it.

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[
extends: [],
import: [],
mappings: [...],
transforms: [
"nadia.token": fn conf ->
[{_, val}] = Conform.Conf.get(conf, "nadia.token")
System.get_env("TELEGRAM_BOT_TOKEN") || val
end],
validators: []
]

In this case we are loading with the conform API the the configured value and return it only if the environment variable is empty. Generating the parameter in this way disallow to have a default in the mapping sections but a workaround would be to chain || to add a default value. I think that this approach is not bad for api key and similar values that you don’t want to checkin with your code (even if are keys of development environments).