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So I've started to study the Elixir language and decided to solve some coding exercise with it in order to better understand some concepts.
Since I liked my solution, I decided to make a post explaining the thought process behind it. Suggestions to further improve the code or to better follow Elixir's conventions are more than appreciated! (:

The Problem

I want to make a function that receives a string as input, supposedly a mathematical expression or some code snippet, and tells me whether all of it's opening parenthesis/brackets/braces have a correct closing match.
Some expressions as examples:

1 * 2 (3 + [4 / 5])
# should return true
9 / [(8 + 7] - 6)
# should return false

Make a plan

Before coding, I think it is a good ideia do solve the problem theoretically, and then try to implement it.
For this exercise, I believe using the stack idea is a very good approach. Basically, a stack is a set of items that have two operations: push and pop.

• The push operation will put a new item in the set
• The pop operation will remove the last pushed item

So the stack follows the "Last In, First Out" dynamic. When I put an item in the stack, this item will be the first one to get out when I start removing them.

How can we use a stack here? Well, my theoretical solution would be to iterate over all the characters in the input string, and if:

• It is an opening char (parens/brackets/braces), put it in a stack
• It is a closing char, check the last item in the stack: If it matches, we pop it and continue the recursion. If it doesn't, the input string is not valid
• If it is something else, ignore

If the whole input string is iterated and I never found an error, this means that all closers had a matching opener, but it doesn't necessarily means the string is valid. This input is an example:

1 + (2 * [3]

All the closers had a matching opener, but not all the openers were closed. This means that if we survive the iteration, we also have to check if our final stack is empty. If it is, all openers were closed, and the input was valid. If it is not, the input was not valid.

The Implementation

Good, we have a plan, so now we have to translate it into Elixir.
For starters, I'm creating a module Parens with a function check that will receive as input a string.
This function check will have to iterate over the string, so we can use the String module's split/2 function. So we have our first piece of code like this:

defmodule Parens do
  def check(expression) do
    expression
    |> String.split("")
  end
end

I'm using the pipe operator |> here, meaning that whatever comes before it, is going to be the first argument of whatever comes after it. In this case, expression is going to be the first argument of String.split.
The second argument is an empty string so we can split the expression at every character. The result of this is a list, like this:

"a + b * (c / d)" |> String.split("")
# ["", "a", " ", "+", " ", "b", " ", "*", " ", "(", "c", " ", "/", " ", "d", ")", ""]

On our theoretical solution, every character but openers/closers were to be ignored, so we can apply a filter in this resulting list. To do that, we can use Enum.filter/2, which receives two arguments.
The first one is something that "is enumerable", which means that this something must implement the Enum protocol.
Luckly, lists do that, so we can pass our resulting list as the first argument to our filter.
The second argument is a function that receives an element of our list and then decide if it should or shouldn't be in the filtered result. More precisely, if this function returns a falsy value (false or nil), then the element will not be in the resulting filtered list. If it returns a truthy value (anything else), it is going to be in the filtered result.
In our case, this function will verify if each element of our list is a parens/brackets/braces and keep it if it is, removing it otherwise. One way to do that is like this:

fn x -> x in ["{", "[", "(", "}", "]", ")"] end

This anonymous function will receive something (x) and see if it belongs to that list, which happens to be a list with only our parens/brackets/braces.
We could also use the capture notation, like this:

&(&1 in ["{", "[", "(", "}", "]", ")"])

The capture notation is similar, but instead of declaring x, we just use &1, &2, &3... for all of our arguments.

Good, so our code right now looks like this:

defmodule Parens do
  def check(expression) do
    expression
    |> String.split("")
    |> Enum.filter(&(&1 in ["{", "[", "(", "}", "]", ")"]))
  end
end

The next step is to iterate over our resulting list and use our "stack strategy". To do that, I decided to create a private function called iterate, where I'll recursively walk over our elements, passing the stack around, until we checked all of our elements. Since I'll need the stack, this function will have an arity of 2, which means it will have 2 arguments.
The first thing I do when I'm thinking about recursion it to write the stop condition. In this case, it should stop once our list of characters is empty. I'll make use of the wonderful pattern matching functionality to do that:

defp iterate([], stack), do: stack

This means that this function will only execute when the first element is an empty list and the second element is whatever it arrives here.
In this case, this function will do nothing and just return the stack, so we can later verify if it is empty or not.

A very important thing to note here is that we also have to pass a stack when we first call the iterate function. Our stack will start empty, so our pipeline will be like this:

def check(expression) do
  expression
  |> String.split("")
  |> Enum.filter(&(&1 in ["{", "[", "(", "}", "]", ")"]))
  |> iterate([])
end

Note: We could also use a default value, but I'll keep it like this.

Now the hard part. After writing a stop condition to our recursion, we have to write the recursive step (or steps).

Let's check our plan again. We have to look at each element, and if it is an opener, we put it in the stack and go on. Let's do this part first.

I love pattern matching, so I'll be using it here again. And to further help us, I'll also use guards to decide if the function will be executed or not:

defp iterate([head | tail], stack) when head in ["{", "[", "("] do
end

This function will only be executed if the variable head belongs to that list of openers. But where is this variable coming from?
We are pattern matching the first argument with [head | tail], so this function will need for the first argument to be a list with a first element. This first element will be bound to head. The rest of the list will be bound to tail. If you are in doubt, open the interactive elixir shell in your terminal (write iex and press enter) and try this code:

[head | tail] = [1,2,3,4]
head
#=> 1
tail
#=> [2,3,4]

[head2 | tail2] = [0]
head2
#=> 0
tail2
#=> []

[head3 | tail3] = []
# Will raise a matching error!

The head and tail are important concepts. The head is the first element of a list, and the tail is the list itself, but without the first element!

Okay, back to our function. What will it do if the character is an opener? It should push it to the stack and continue iterating. We can do that simply by calling iterate again with the right arguments.
Since we already checked the first element, we can pass the list without the first element. That is precisely what tail is! Good, but how can we push the element to the stack?
Elixir offers a good way to put an item in a list, and it is simply like this:

[new_element | stack]

This is a list which its head is new_element, and its tail is the stack. You can always go to iex and make some experiments:

[1 | 9,8,7]
#=> [1,9,8,7]

So our function looks like this:

defp iterate([head | tail], stack) when head in ["{", "[", "("],
  do: iterate(tail, [head | stack])

When it is an opener, we simply call iterate with tail and pass the stack with a newly pushed head item.

Good! Now that we have this part done, let's check the plan again.

We already know how to stop the recursion;
We already took care of the "ignore if not opener/closer" issue;
We already know how to push an opener to the stack and continue;

We now have to do something when the element is a "closer". When that happens, the idea was to look at the last added item of our stack, which happens to be the head of stack.

This part of the code was heavily refactored, but I'll tell how I did it first time. For each possible closer, which are }, ] and ), I made its single pattern matched function:

defp iterate(["}" | tail], stack)
defp iterate(["]" | tail], stack)
defp iterate([")" | tail], stack)

And for each one of those, I made a simple if/else statement:

defp iterate(["}" | tail], stack) do
  if hd(stack) == "{" do
    iterate(tail, tl(stack))
  else
    false
  end
end

First of all, I used the functions hd and tl, which returns the head and the tail of a list, respectively. It would be the same as this:

defp iterate(["}" | tail], [stack_head | stack_tail]) do
  if stack_head == "{" do
    iterate(tail, stack_tail)
  else
    false
  end
end

Then, since this function will only execute if the element is exactly }, I'm verifying if the last added item in the stack (its head) is {, which would be a match.
If it is, we have to pop it. The "popped" version of the stack is simply its tail, so that's why we don't pass the stack to the next iteration, but rather the stack_tail.
If it doesn't match, we know that the input string is not valid, so I'm returning a false here.

The else part is not really needed, because in order to go inside it, the comparison should be falsy. When that happens, an if statement alone would return nil, but I'll keep it with this else for now. We can refactor later.

This looks like a valid "alpha version" of our iterate function. What is missing is verifying if the stack is empty or not after the stop condition. A way to do that is to add a comparison with [] in the stop condition or in the pipeline of our check function. Enum.empty? would also work, as it does exactly what its name suggests. I'll show the "comparison with []" first because I can talk briefly about the Kernel module.

The Kernel module has all the functions we can use "natively", without calling a module. So if you want so sum two integers, you can do both:

1+2
#=> 3
Kernel.+(1,2)
#=> 3

The == comparison is also part of the Kernel module, so we can do this:

  def check(expression) do
    expression
    |> String.split("")
    |> Enum.filter(&(&1 in ["{", "[", "(", "}", "]", ")"]))
    |> iterate([])
    |> Kernel.==([])
  end

This will take whatever returns from the iterate call and use it as the first argument of Kernel.==. The second argument is [], so we are going to return the comparison of the result of iterate with [].
If the input is invalid, the iterate may return false, and then the comparison with [] will also be false.
Another possibility is that the iterate will return a not empty stack. Then, the comparison with [] will be false as well.
This actually solves it, but for me it is really ugly.

For starters, sometimes iterate returns a boolean, and sometimes it returns a list, which could be empty or not.
Now that I wrote about Kernel a little bit, I think we should not use it in this case (:
To avoid it, we can put this comparison in our stop condition, like this:

defp iterate([], stack), do: stack == []

Another way is to pattern match the second argument as well!

defp iterate([], []), do: true
defp iterate([], _stack), do: false

If the first argument is an empty list and the second argument is also an empty list, we return true.
If the first argument is an empty list and the second argument is anything (the _ denotes that this variable will not be used), we return false.
Note: We could simply use _ instead of _stack, but I think it is nice to put some name to it in the name of readability, since it is not obvious what the second argument could be

Okay, so this apparently works, right?

If we try to run our code now, we are going to sometimes get an error!
To test it, we can use our module inside iex. To do that, save the code in a file, for example, parens.ex. Now, run iex parens.ex. You'll be able to use the Parens module we just created inside iex!
If you try to check a string where a closer would be found before any opener, the code would try to get the head of an empty stack, which raises an error. You can verify it:

Parens.check("a}")
#> Raises error!

Parens.check("(a}")
#=> false

We can fix this by checking if the stack is empty before trying to get its head.

Or...

We could simply pattern match the second argument to empty list when we have a closer, like this:

defp iterate([head | _], []) when head in ["}", "]", ")"], do: false

The guard will ensure that head is a closer, and when the stack is an empty list, we just return false before trying to get the head of an empty list (which raises an error).

The first version of our solution could then be this:

defmodule Parens do
  def check(expression) do
    expression
    |> String.split("")
    |> Enum.filter(&(&1 in ["{", "[", "(", "}", "]", ")"]))
    |> iterate([])
  end

  defp iterate([], []), do: true
  defp iterate([], _stack), do: false

  defp iterate([head | tail], stack) when head in ["{", "[", "("],
    do: iterate(tail, [head | stack])

  defp iterate([head | _], []) when head in ["}", "]", ")"],
    do: false

  defp iterate(["}" | tail], stack) do
    if hd(stack) == "{" do
      iterate(tail, tl(stack))
    else
     false
    end
  end

  defp iterate(["]" | tail], stack) do
    if hd(stack) == "[" do
      iterate(tail, tl(stack))
    else
     false
    end
  end

  defp iterate([")" | tail], stack) do
    if hd(stack) == "(" do
      iterate(tail, tl(stack))
    else
     false
    end
  end
end

Nice! So now our code actually works!
But it doesn't really give me good vibes...

Refactor!

First, we are using lists like ["{", "[", "("], ["{", "[", "("] and ["{", "[", "(", "}", "]", ")"], which could be extracted to module attributes.
Module attributes are values that can be used by any method in a module. To define them, we can write @attribute_name and then its value. We can do it like this:

defmodule Parens do
  @opener ["{", "[", "("]
  @closer ["}", "]", ")"]
end

This is a nice little addition, so now we can rewrite our guards and filter like this:

Enum.filter(&(&1 in ["{", "[", "(", "}", "]", ")"]))
Enum.filter(&(&1 in @opener || &1 in @closer))

defp iterate([head | tail], stack) when head in ["{", "[", "("]
defp iterate([head | tail], stack) when head in @opener,

defp iterate([head | _], []) when head in ["}", "]", ")"], do: false
defp iterate([head | _], []) when head in @closer, do: false

But we still have 3 big functions that are basically the same, which are the ones that decide what to do when the element we are checking is a closer character.
I'll write what I did and then explain it:

defmodule Parens do
  @pairs %{"{" => "}", "[" => "]", "(" => ")"}

  defp iterate([head | tail], [stack_head | stack_tail]) when head in @closer,
    do: @pairs[stack_head] == head && iterate(tail, stack_tail)
end

So first, I created a @pairs module attribute which is a map. Each key of the map is an opener character and it maps to a closer character (maps are like hashes and dictionaries, if you are coming from ruby or python).
Then, I made an iterate/2 function that has a guard. This guard will ensure that head variable (the first element of our list) is a closer (so it is one of ), ] or }).
I also used a && boolean operation here:

@pairs[stack_head] == head && iterate(tail, stack_tail)

If the left side is a falsy value, then this value is returned and the right side is not executed (which stops our recursion).
If the left side is truthy, then whatever is at the right side is returned. In this case, the right side is a function call, continuing the recursion.

Now I'll look at @pairs[stack_head]. Remember that @pairs is a map, so @pairs["{"], for example, returns "}".
If whatever is the head of our stack is an opener, then it maps to some closer (@pairs[stack_head], then, is some closer). If this closer equals to head (the element we are checking itself), then the comparison returns true, which will then return the right side of the &&, continuing our recursion!
If not, then the comparison will return false, and not execute the right side of the &&.
So this is enough to check if the parens are matching and stop the recursion otherwise.

So our second version of the program is:

defmodule Parens do
  @pairs %{"{" => "}", "[" => "]", "(" => ")"}
  @opener ["{", "[", "("]
  @closer ["}", "]", ")"]

  def check(expression) do
    expression
    |> String.split("")
    |> Enum.filter(&(&1 in @opener || &1 in @closer))
    |> iterate([])
  end

  defp iterate([], []), do: true
  defp iterate([], _stack), do: false

  defp iterate([head | tail], stack) when head in @opener,
    do: iterate(tail, [head | stack])

  defp iterate([head | _], []) when head in @closer, do: false

  defp iterate([head | tail], [stack_head | stack_tail]) when head in @closer,
    do: @pairs[stack_head] == head && iterate(tail, stack_tail)
end

That's it for today

Thanks for reading this. I'm enjoying Elixir very much. Pattern matching is quite powerful and fun.
Please correct any mistakes I might have made (:
I hope you learned something today, and have a good day.