Concatenation of Strings fails with {ArgumentError}

Of course it does. All (well, most) Enum functions return a list. You can not use <> on Lists.

If I understand you correctly, you want this:

str_to_hash = params
|> Enum.map({k, v} -> "#{k}#{v}" end)
|> Enum.concat([sc_request.secret_key])
|> Enum.join

(untested)

Thank you, NobbZ, that is exactly what I wanted.

Added just the fn keyword and the function now looks like this:

  defp calculate_v4_checksum(sc_request, params) do
	str_to_hash = params
				  |> Enum.map(fn {k, v} -> "#{k}#{v}" end)
				  |> Enum.concat([sc_request.secret_key])
				  |> Enum.join
	:crypto.hash(sc_request.algorithm, str_to_hash)
	|> Base.encode16(case: :lower)
  end

But back to my original question: why this fails(aren’t both values strings ?):

IO.puts (str_to_hash <> sc_request.secret_key)

PS: You can simulate your ordered map roughly like this (and also do it single pipe):

params
|> Enum.sort()
|> Enum.map(fn {k, v} -> "#{k}#{v}" end)
|> Enum.concat([sc_request.secret_key])
|> Enum.join
|> (&:crypto.hash(sc_request.algorithm, &1)).()
|> Base.encode16(case: :lower)

As I already said in my first answer, Enum.map returns a list, due to the function you passed into it, that was a list of String.t, but a list of strings is not a string. <> does really only work for binaries (and therefore for strings as well).