Valid JSON is valid YAML. Any conforming YAML parser should be able to parse JSON. So if you don’t have any specific needs for the YAML format, doing something like:

"---\n" <> Poison.encode!(data)

will produce something any YAML parser will be able to process.

@50kudos For what it’s worth, I found myself in a similar situation. I could parse YAML with yaml-elixir but didn’t have a way to write. So I wrote a short YAML writer myself.

def to_yaml(yaml_map, indentation \\ "") do
    yaml_map
    |> Map.keys
    |> Enum.map(
      fn key ->
        value = Map.fetch!(yaml_map, key)
        cond do
          is_bitstring(value) -> "#{indentation}#{key}: #{value}"
          is_number(value) -> "#{indentation}#{key}: #{value}"
          is_map(value) -> "#{indentation}#{key}:\n#{to_yaml(value, "#{indentation}  ")}"
        end
      end
    )
    |> Enum.join("\n")
end

You use it as follows:

YamlElixir.read_from_file!("file.yaml")
|> Map.put(:some_key, %{some_nested_key: 123})
|> to_yaml

It’s not perfect but it suits my purposes (reading and writing the app.yaml configuration file in Google App Engine).

Thought I’d share to help others who come here while searching, like I did!

and there is a good reason for that: YAML is not a standardized format. In fact each and every implementation of YAML out there varies slightly. So when you generate YAML config file from most popular PHP library, it may or may not work as expected in most popular Ruby library etc.

I think @josevalim had a strong opinion on not using YAML in Elixir, and this sentiment has been picked up by the community, since the not any sort of standard argument is really good one. As a result we have a few parsers, since reading some legacy config files may be necessary - and no generators that I can see.

I’d probably generate some intermediate data format from my data (JSON?) and convert it to YAML with some external to Elixir tool (Ruby?).

You see, CSV happens to be implemented in the wildest ways, yet if customers / stakeholders of my project require a CSV export, they won’t be happy with an answer like yours.

I can understand that José does not want to use YAML FOR Elixir (e.g. he said you should not use it for Elixir configuration, and I totally agree with that). My use case is a different one. I’m building an API that needs to return a YAML file, because it is needed as a config for a separate application that I cannot change.

This, in my books, would qualify as an ugly workaround. Now I can’t say in the docs: “just curl https://foo/bar.yml > bar.yml” and you’re done! Now I’d need to deliver a script or tool to make the conversion and have the user possibly have the required runtime (or use Go, but then you have to publish a binary for every major desktop OS).

All this discussion about YAML being terrible is nice and entertaining, until you hit the real world. I’m not yet convinced of why not having a YAML generator in Elixir might be a good idea.