First of all the body parameter should be a string, not a list.

Second thing is that Jason.encode/2 returns response in format {:ok, binary}, you either pattern match on response or use the bang variant Jason.econde!/2.

The third thing is that jason doesn’t know how to encode tuples, since json format doesn’t have this concept, so you have to use a map instead:

body = %{language: "sql", command: "select from Profile"}
body_binary = Jason.encode!(body)

This is absolutely not true. Elixir from conception was created to have a good documentation infrastructure in mind and all mature libraries have great documentation. It is a lot to take in as a beginner as @benwilson512 said, because there are a lot of things to unpack.

A short description on how you should read documentation is the following:

1. Open the function documentation on hexdocs for HTTPoison.post/4
2. Look at the specs of the function, in our case we have the following:

post(url, body, headers \\ [], options \\ [])

# spec
post(binary(), any(), headers(), Keyword.t()) ::
{:ok,
 HTTPoison.Response.t()
 | HTTPoison.AsyncResponse.t()
 | HTTPoison.MaybeRedirect.t()}
| {:error, HTTPoison.Error.t()}

3. For headers, if you follow the link you will get the following spec:

headers() ::
  [{atom(), binary()}]
  | [{binary(), binary()}]
  | %{required(binary()) => binary()}
  | any()

This states clearly that you can create headers in the following formats:

["Authorization": authString, "Content-Type": "application/json"] 
[{"Authorization", authString}, {"Content-Type" "application/json"}]
%{"Authorization" => authString, "Content-Type" => "application/json"}  

4. Now the body parameter is set as any, however if you follow the description of the parameter from the function, you will be redirected to request/5 and then to HTTPoison.Request where there is a clear description of the format:

:body:
binary, char list or an iolist
{:form, [{K, V}, …]} - send a form url encoded
{:file, “/path/to/file”} - send a file
{:stream, enumerable} - lazily send a stream of binaries/charlists

Could this documentation be improved? definetly, more examples could be added also, nothing is perfect
Is it missing vital documentation? absolutely no

And that is all you need to know, as long as you respect these specs, you requests should work as expected.

My advice is that if you don’t want to keep shooting yourself in the leg, take some time to learn elixir syntax, preferably read a book that covers the philosophy of the language, then dive into more complex things like phoenix, as that is a full fledged mature framework that is not meant to be beginner friendly, but to get things done.

One very important thing - Elixir uses snake case naming. If you don’t follow that, formatting might have issues.