Comparison of single-quoted and double-quotes Strings

The problem with your compare function is, that it will crash as soon as you pass something that is not 'C'.

It should work besides of that. What is it you see instead?

Character data type is absolutely the same as Unsigned Integer type for most programming languages.
The same is for Elixir.

So, 'AZERTY' is list of integers, that are just looks like letters (or string). For example, you can add one more element to the list like this:

iex> 'AZERY' ++ [0]
[65, 90, 69, 82, 89, 0]

And as expected you see the list of integer (because Elixir cant convert 0 to a letter)

As you see here, A = 65. So, what is 'A'? OF cource it’s list with one element

iex> 'A' == [65]   
true

Now, the question is:

How can we represent char, if we don’t want to look at the ASCII table every time, to see that A is 65?

Using ? syntax sugar!

iex> ?A
65

Now, you can do:

defp convert(letter) when ?C == letter do
    IO.puts("+++++++ equal!!! ++++++++++ ")
end

I’m trying to catch the letters as follows:

def to_rna(dna) do
    Enum.each(dna, &convert(&1))
  end

  defp convert('G') do
    'C'
  end

  defp convert('C') do
    'G'
  end

  defp convert('T') do
    'A'
  end

  defp convert('A') do
    'U'
  end

  defp convert(letter) do
    IO.puts("Unknown letter")
  end

And the test:

assert RNATranscription.to_rna('G') == 'C'

fails as follows:

Assertion with == failed
     code:  assert RNATranscription.to_rna('G') == 'C'
     left:  :ok
     right: 'C'
     stacktrace:
       rna_transcription_test.exs:13: (test)

Can’t understand what’s really wrong

Enum.each returns :ok, you need Enum.map here.
It’s good idea to ask these questions from your Exercism mentor, not on forum

'C' == letter

is simply an expression the either evaluates to true or false

when 'C' == letter

Here that expression is part of a guard, i.e. if

• true → match
• false → match fails

def convert(letter) when 'C' == letter do
    IO.puts("+++++++ equal!!! ++++++++++ ")
end

Is equivalent to

def convert('C') do
  IO.puts("+++++++ equal!!! ++++++++++ ")
end

Given the absence of other function clauses a failed match will result in a program crash. To avoid that:

def convert('C') do
  IO.puts("+++++++ equal!!! ++++++++++ ")
end
def convert(_) do
  IO.puts("+++++++ not equal!!! ++++++++++ ")
end

This function convert/1 is one function with two function clauses (function heads).

OK, it is getting clearer now. Thank you very much for such a detailed explanation, really useful
Taking into account that Enum.each returns just :ok and map returns a list, I think a better solution would be to return a corresponding matching letter, so in pseudo language it would read:

if 'C' => return 'G'
if 'G' => return 'C'
etc...

Look at case do

Also remember that everything is an expression (there are no statements) so everything evaluates to some kind of value.

This is why there is no return statement - functions have to evaluate to a value just like expressions, so they automatically “return” the last value they evaluated.

But you’re still comparing lists of 1 character, and returning lists of 1 character, not single characters.

As I’m getting a list of characters (either of just one, A, or a sequence like AZERTY), I think I have to

• make them match one by one to make a translation to other characters according to the translation table (like A should be replaced with T, Z → with U, etc.)
• build another chain of translated characters and return it.

Yeah, it is like in Ruby methods where the last evaluated expression is returned from a method.