In the previous articles, we have implemented the Toy Robot (you can find it here Part 1 and Part 2). This time we will improve the implementation and turn it into a console application. In that application, we will be able to run the simulator and give commands to the robot.

If you would like to follow these steps on your own, you can use the code as a starting point. Be sure to use v1.0 tag for that.

## Start and welcome

Let’s implement the first step of the application. We will be able to run it and display a welcome message.

To turn the app into executable we can use Erlang’s escript command, which is supported by Elixir as well. Before we can “build” that executable we need to describe which module will we use as a starting point for our application. To do that we need to extend project function from mix.exs file by adding escript option:

defmodule ToyRobot.Mixfile do
use Mix.Project

def project do
[
app: :toy_robot,
# ...
escript: escript() # <- That is a new line
]
end

# ...

defp escript do
[main_module: ToyRobot.CLI]
end
end


Now we need to define ToyRobot.CLI module. The module should contain main/1 function, which will be the starting point of the application. For now, let’s display our welcome message and quit.

defmodule ToyRobot.CLI do
def main(_args) do
IO.puts("Welcome to the Toy Robot simulator!")
end
end


We are ready to build the application and use it from a command line. We will use mix escript.build to do that.

→ mix escript.build
Compiling 3 files (.ex)
Generated toy_robot app
Generated escript toy_robot with MIX_ENV=dev


The build command will use the name of the application as a file name. Now we can run it.

→ ./toy_robot
Welcome to the Toy Robot simulator!


We can also copy that script and run it on any other machine which has Erlang installed.

→ cp toy_robot ~/Desktop/
→ ~/Desktop/toy_robot
Welcome to the Toy Robot simulator!


Congratulations! First steps are done. We can run the app from the terminal. But we don’t want it to only write a message and quit, do we? Instead, we want to give commands to our robot.

Let’s try to implement our first command.

## The quit command

It would like to start from the simplest command. First, we need a plan that we need to do in that section. We want our application to display a welcome message followed by help instructions. If a new user starts using our simulator it would be nice to explain to him what commands he can run.

Next, we need the program to listen to the commands and wait for user’s input.

If user types “quit” we want to close the program. We also want to display an error message in case a command does not support the simulator.

Let’s see the extended version of the code and slowly walk through it.

defmodule ToyRobot.CLI do
def main(_args) do
IO.puts("Welcome to the Toy Robot simulator!")
print_help_message()
end

@commands %{
"quit" => "Quits the simulator"
}

IO.gets("\n> ")
|> String.trim
|> String.downcase
|> execute_command
end

defp execute_command("quit") do
IO.puts "\nConnection lost"
end

defp execute_command(_unknown) do
IO.puts("\nInvalid command. I don't know what to do.")
print_help_message()

end

defp print_help_message do
IO.puts("\nThe simulator supports following commands:\n")
@commands
|> Enum.map(fn({command, description}) -> IO.puts("  #{command} - #{description}") end)
end
end


At first, we have extended our main function to display the help message right after welcome message. The help message provides the list of all available commands user can use. At that moment there is only “quit” command.

Right after display of welcome and help messages we are triggering the receive_command function. This function listens for user’s input. Once we get the input we cut the line breaks “\n” and transform it into a lowercase string. After those transformations, we pass it to an execute_command function.

If a user provides us “quit” string we display a farewell message and quit. That means the defp execute_command("quit") function is called.

For any other input, we will display an invalid command message followed by the help. We also call the receive_command function again, so a user can repeat the process.

## The placement command

Once we finished with the backbone of simulator’s interface we can proceed and implement the placement command.

First, let’s add it to the list of supported commands.

@commands %{
"quit" => "Quits the simulator",
"place" => "format: \"place [X,Y,F]\". " <>
"Places the Robot into X,Y facing F (Default is 0,0,North). " <>
"Where facing is: north, west, south or east."
}


Then we need to implement execute_command function for the default case. If we pass no arguments to ToyRobot.place function it would place it into a default position.

defp execute_command("place") do
ToyRobot.place
end


Let’s check it

→ mix escript.build && ./toy_robot
Compiling 1 file (.ex)
Generated escript toy_robot with MIX_ENV=dev
Welcome to the Toy Robot simulator!

The simulator supports following commands:

place - format: "place [X,Y,F]". Places the Robot into X,Y facing F (Default is 0,0,North). Where facing is: north, west, south or east.
quit - Quits the simulator

> place
> quit

Connection lost


Great. Now we need our place command to support arguments. In that case, we need to change the way of calling our execute_command functions. At first, we need to split the command itself and its attributes. We can do it by adding String.split(" ") step into receive_command.

defp receive_command do
IO.gets("> ")
|> String.trim
|> String.downcase
|> String.split(" ") # <- additional step here
|> execute_command
end


Now instead of a string, the execute_command function will receive a list. The first element of that list would contain the command and the second - raw parameters if there are any. That means we need to update rest of our functions to support that.

defp execute_command(["place"])

defp execute_command(["quit"])


Now, go build and run the app to check if it still works. After that, we are ready to proceed with the implementation.

defp execute_command(["place" | params]) do
{x, y, facing} = process_place_params(params)

case ToyRobot.place(x, y, facing) do
{:ok, _robot} ->
{:failure, message} ->
IO.puts message
end
end

defp process_place_params(params) do
[x, y, facing] = params |> Enum.join("") |> String.split(",") |> Enum.map(&String.trim/1)
{String.to_integer(x), String.to_integer(y), String.to_atom(facing)}
end


This function will be called if a user enters place command with arguments. We checking it by explicitly comparing with place command and capture the params.

The ToyRobot.place/3 function expects coordinates x and y to be a type of Integer and facing as an Atom. That means we need to transform our string of parameters into those values. We do that using process_place_params/1 function. We are joining the params just in case there is more than one element in the list. Then we split the string by commas. Then we cut the garbage out. Then we turn it into a tuple with valid data.

Once we get our params parsed. We call ToyRobot.place/3 and passing those params into it. If that call was not successful we would display the error message. Either way, we listen to a next user’s command.

### The Report command

As usual, let’s begin by adding report to the list of available commands.

@commands %{
# ...
"report" => "The Toy Robot reports about its position"
}


In order to get reports from the robot, we need to call ToyRobot.report/1 function. This function requires us to pass robot’s position. Which we didn’t know yet. We are placing our robot, but immediately throw away all the knowledge about its position.

Let’s fix that. Once we receive the position of the robot from the place command we will pass it through every time. In that case, we will keep the position of the robot.

defp receive_command(robot \\ nil) do
IO.gets("> ")
|> String.trim
|> String.downcase
|> String.split(" ")
|> execute_command(robot)
end


Our receive_command receives the robot as an argument and passes it to the execute_command function. The robot is nil by default because it was not placed yet.

Let’s update all the versions of our execute_command functions to receive robot argument, and pass it through if needed.

defp execute_command(["place"], _robot) do

defp execute_command(["place" | params], _robot) do

defp execute_command(["quit"], _robot) do

defp execute_command(_unknown, robot) do


What kind of report would we get if we ask non-placed robot? I think we would respond with an error.

defp execute_command(["report"], nil) do
IO.puts "The robot has not been placed yet."
end


In case the robot is in position, we can display its coordinates:

defp execute_command(["report"], robot) do
{x, y, facing} = robot |> ToyRobot.report
IO.puts String.upcase("#{x},#{y},#{facing}")

end

> place 1, 2, south
> report
1,2,SOUTH


Cool. The major part is done.

### Rotate and move

Now our code is well shaped and we can easily extend it with rotate and move functionality.

As usual, we need to define it as an available command by extending @commands attribute. We will add left command:

@commands %{
# ...
"left" => "Rotates the robot to the left"
}

defp execute_command(["left"], robot) do
end


We are receiving a position of the robot, we rotate the robot and pass it through to listen to the following commands.

The result is:

> place
> report
0,0,NORTH
> left
> report
0,0,WEST


Implementation of the rotation to the right and movement would be pretty similar:

@commands %{
# ...
"right" => "Rotates the robot to the right",
"move"  => "Moves the robot one position forward"
}

defp execute_command(["right"], robot) do
end

defp execute_command(["move"], robot) do
end

> place
> report
0,0,NORTH
> move
> right
> move
> left
> move
> report
1,2,NORTH


That is it. We have made available all robot’s functionality in our command line application.

There is one more thing left. The subject of writing command line application would not be complete without mention how to work with parameters. Let’s cover that topic.

## Accepting parameters

Our main/1 function receives attribute. Which we were not using until now. This attribute contains the list of parameters if they were passed. It would contain an empty list [] if there are no parameters passed.

Let’s extend the program to provide a help message if a user runs our program with --help attribute like this:

./toy_robot --help


In that case, our args would hold a ["--help"] value. One way can be just to fetch the "--help" value our of the list using pattern matching. But Elixir ships with the OptionParser.parse/2 which can help us to parse params.

Let’s fire an iex and play with it a little bit. If we run a console app with those parameters:

./toy_robot --help instructions.csv --something=else


The args would contain following list

["--help", "instructions.csv", "--something=else"]


By passing it to the OptionParser.parse/2 we would get:

> OptionParser.parse(["--help", "instructions.csv", "--something=else"])
{[help: "instructions.csv"], [], [{"--something", "else"}]}


The result contains a tuple with 3 lists in it. The first list is a “parsed” arguments, second is the rest of arguments and the third is a list of invalid arguments.

That result does not look how we want it. We can pass additional parsing options as the second argument to an OptionParser.parse/2 function. Those options are some kind of rules.

For example. We understand that --help option is a boolean, it was either requested or not. We can specify it as following:

> OptionParser.parse(["--help", "instructions.csv", "--something=else"], switches: [help: :boolean])
{[help: true], ["instructions.csv"], [{"--something", "else"}]}


Now we get it as a boolean. The “instructions.csv” now is in the rest parameters, but “something” is still in the invalid. We can fix that as well.

> OptionParser.parse(["--help", "instructions.csv", "--something=else"], switches: [help: :boolean, something: :string])
{[help: true, something: "else"], ["instructions.csv"], []}


Now it’s properly parsed.

You can find a way more options in the documentation of the OptionParser.parse/2. Now let’s proceed and implement our help functionality.

So we only care about --help option to display a help message once it exists. Otherwise, we launch the simulator.

def main(args) do
args |> parse_args |> process_args
end

def parse_args(args) do
{params, _, _} =  OptionParser.parse(args, switches: [help: :boolean])
params
end

def process_args([help: true]) do
print_help_message()
end

def process_args(_) do
IO.puts("Welcome to the Toy Robot simulator!")
print_help_message()
end


We have changed our main/1 function. We parse the arguments. We transform them into the format we want. We have now two versions of process_args/2 function which we use to either show a welcome message or proceed and start the simulator.

Go ahead, build the app and run ./toy_robot --help command. You will see a help message and the program will be terminated.

## Wrapping Up

Let’s take a quick look back and see what did we achieve here.

Now we know how to build a console application and how to work with command line arguments. On top of that, we have extended the Toy Robot simulator which we can play with it in the interactive mode.

The complete list of the changes you can find on a GitHub page of the project.

See you next time, where we will implement new stuff. Don’t wanna miss a thing? Then subscribe in the form below.