https://elixir-lang.org/getting-started/module-attributes.html#as-constants

• Why does that tag mention A, C, G, T preceded by a question mark?

https://elixir-lang.org/getting-started/binaries-strings-and-char-lists.html#utf-8-and-unicode

i.e.

• What is the best source where I can have a more detailed explanation than in elixirschool (i.e.) with examples?

The “best” varies from person to person. But I don’t think you can go wrong with

• Elixir - Getting Started Pages (1-23) and then
• Elixir - MIX AND OTP (1-10)

First pass:

# file: nucleotide_count.exs
defmodule NucleotideCount do

  def counthelper(c, strand, nucleotide) do
    [hd | tl] = strand
    cond do
      hd == nucleotide -> counthelper(c + 1, tl, nucleotide)
      length(tl) != 0 -> counthelper(c, tl, nucleotide)        # 2.) Need (length tl) or length(tl)
      true -> c
    end
  end

  def count(strand, nucleotide) do
    counthelper(0, strand, nucleotide)
  end
end # 1.) added missing module "end"

IO.inspect(NucleotideCount.count('AATAA', ?A))
IO.inspect(NucleotideCount.count('AATAA', ?T))

But still:

$ elixir nucleotide_count.exs
** (MatchError) no match of right hand side value: []
    nucleotide_count.exs:5: NucleotideCount.counthelper/3
    nucleotide_count.exs:18: (file)
    (elixir) lib/code.ex:677: Code.require_file/2
$

i.e. [hd | tl] = strand the match fails when the list is empty - the code never gets to length(tl) != 0

quick fix: Pattern Matching

# file: nucleotide_count.exs
defmodule NucleotideCount do

  def counthelper(c, [], _) do
    c # ran out of strand - return count
  end
  def counthelper(c, strand, nucleotide) do
    [hd | tl] = strand
    cond do
      hd == nucleotide -> counthelper(c + 1, tl, nucleotide)
      length(tl) != 0 -> counthelper(c, tl, nucleotide)
      true -> c
    end
  end

  def count(strand, nucleotide) do
    counthelper(0, strand, nucleotide)
  end
end

IO.inspect(NucleotideCount.count('AATAA', ?A))
IO.inspect(NucleotideCount.count('AATAA', ?T))

$ elixir nucleotide_count.exs
4
1
$

But we can do better …

# file: nucleotide_count.exs
defmodule NucleotideCount do

  def counthelper(c, [], _),
    do: c                                                        # ran out of strand - return count
  def counthelper(c, [hd|tl], nucleotide) when hd == nucleotide, # 1. move pattern match to function head
    do: counthelper(c + 1, tl, nucleotide)                       # 2. extend pattern with a guard
  def counthelper(c, [_|tl], nucleotide),                        # 3. here we know "hd" isn't the "nucleotide"
    do: counthelper(c, tl, nucleotide)                           # we don't care about the "hd" so we use "_" instead

  def count(strand, nucleotide),
    do: counthelper(0, strand, nucleotide)

end

IO.inspect(NucleotideCount.count('AATAA', ?A))
IO.inspect(NucleotideCount.count('AATAA', ?T))

guards
named functions and do/end blocks

The whole thing may look a bit more familiar this way:

# file: nucleotide_count.exs
defmodule NucleotideCount do

  def counthelper(c, strand, nucleotide) do
    case strand do
      [x|xs] when x == nucleotide ->
        counthelper(c+1,xs,nucleotide)
      [_|xs] ->
        counthelper(c,xs,nucleotide)
      _ ->
        c
    end
  end

  def count(strand, nucleotide),
    do: counthelper(0, strand, nucleotide)

end

IO.inspect(NucleotideCount.count('AATAA', ?A))
IO.inspect(NucleotideCount.count('AATAA', ?T))

And then there is

# file: nucleotide_count.exs
defmodule NucleotideCount do

  def count(strand, nucleotide) do
    n_accumulate = fn
      x,c when x == nucleotide -> c + 1
      _,c -> c
    end
    List.foldl(strand, 0, n_accumulate)
  end

end

IO.inspect(NucleotideCount.count('AATAA', ?A))
IO.inspect(NucleotideCount.count('AATAA', ?T))

List.foldl/3
Enum.reduce/3

Defining Multiple Clauses In An Anonymous Function