Question

I have an empty list, and, in a special case, want to append a tuple to the list. How can I do that?

I tried several approaches:

case ReqFilePath of
    "style.css" ->
        ResponseHeaders = [{"Content-Type", "text/css"}];
    _Else ->
        ResponseHeaders = []
end,


case filelib:is_file(File) of
    true ->
        {ok, Content} = file:read_file(File),
        {output, Content, ResponseHeaders}; % Complains ResponseHeaders is not safe
    false ->
        not_found
end.

This does not work either, since the variable is already set. When I initialize ResponseHeaders = [] first, and then try to add a value to it

ResponseHeaders = lists:append(ResponseHeaders, [{"Content-Type", "text/css"}]);

I get a match error. How do you do this normally in Erlang?

Answer 1

The usual way to do this in erlang is just to use another variable:

ResponseHeaders = [{"Content-Type", "text/css"}],
[...]
ResponseHeaders2 = ResponseHeaders ++ [{new_thing}]

Or you could make a function that builds the ResponseHeaders for you:

ResponseHeaders = build_headers(ReqFilePath).

build_headers("skin.css") -> [{"Content-Type", "text/css"}];
build_headers(_) -> [].

Don’t forget to change your mindset to the erlang way! 😉