String.char error - (Protocol.UndefinedError) protocol String.Chars not implemented

Questions / Help
1

this code is from a course from pragmaticstudio

def parse(request) do
#TODO: parse the request string into a map:
first_line = request |> String.split("\n") |> List.first
[method, path, _] = String.split(first_line, " ")
%{ method: method, path: path, resp_body: “”}
end

this line:
%{ method: method, path: path, resp_body: “”}
gives me the error:

== Compilation error in file lib/servy/handler.ex ==
** (Protocol.UndefinedError) protocol String.Chars not implemented for %{method: "GET", path: "/wildthings", resp_body: ""} of type Map
    (elixir 1.12.3) lib/string/chars.ex:3: String.Chars.impl_for!/1
    (elixir 1.12.3) lib/string/chars.ex:22: String.Chars.to_string/1
    (elixir 1.12.3) lib/io.ex:712: IO.puts/2
    (elixir 1.12.3) lib/kernel/parallel_compiler.ex:319: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/7

but the code does not give an error on their screen.

the full error message:

(Protocol.UndefinedError) protocol String.Chars not implemented for %{method: "GET", path: "/wildthings", resp_body: ""} of type Map
2

The error appears to be caused by passing the result of parse to IO.puts; can you show that code?

FWIW, IO.inspect is usually a better tool for printing things mid-calculation, as it can show maps/tuples/PIDs/etc that don’t work with IO.puts.

2 Likes
3

that is the entire code bit,
however when I call the next function using the IO.inspect I still get the same thing:

  def route(conv) do
    IO.inspect(conv)
  end

the error from the route function:

** (Protocol.UndefinedError) protocol String.Chars not implemented for %{method: "GET", path: "/wildthings", resp_body: ""} of type Map
    (elixir 1.12.3) lib/string/chars.ex:3: String.Chars.impl_for!/1
    (elixir 1.12.3) lib/string/chars.ex:22: String.Chars.to_string/1
    (elixir 1.12.3) lib/io.ex:712: IO.puts/2

and here’s the thing, this is the 3rd or fourth time I’m going thru this chapter for reviewing and practicing and did not have the problem before, but I was using a windows box, now I’m using a mac with the latest elixir version if that makes any difference.
when I comment the IO.inspect on the route function, the error goes away… bummer

PS: thanks for your response

4

in looking at the protocol implementation code:

defmodule Protocol.UndefinedError do
  defexception [:protocol, :value, description: ""]

  @impl true
  def message(%{protocol: protocol, value: value, description: description}) do
    "protocol #{inspect(protocol)} not implemented for #{inspect(value)} of type " <>
      value_type(value) <> maybe_description(description) <> maybe_available(protocol)
  end

  defp value_type(%{__struct__: struct}), do: "#{inspect(struct)} (a struct)"
  defp value_type(value) when is_atom(value), do: "Atom"
  defp value_type(value) when is_bitstring(value), do: "BitString"
  defp value_type(value) when is_float(value), do: "Float"
  defp value_type(value) when is_function(value), do: "Function"
  defp value_type(value) when is_integer(value), do: "Integer"
  defp value_type(value) when is_list(value), do: "List"
  defp value_type(value) when is_map(value), do: "Map"
  defp value_type(value) when is_pid(value), do: "PID"
  defp value_type(value) when is_port(value), do: "Port"
  defp value_type(value) when is_reference(value), do: "Reference"
  defp value_type(value) when is_tuple(value), do: "Tuple"

  defp maybe_description(""), do: ""
  defp maybe_description(description), do: ", " <> description

  defp maybe_available(protocol) do
    case protocol.__protocol__(:impls) do
      {:consolidated, []} ->
        ". There are no implementations for this protocol."

      {:consolidated, types} ->
        ". This protocol is implemented for the following type(s): " <>
          Enum.map_join(types, ", ", &inspect/1)

      :not_consolidated ->
        ""
    end
  end
end

the protocol definition code.

5

That is by design, you shouldn’t be trying to print a map. The bug is that somewhere in your code you are trying to treat a map as a map. You have a parse function, what do you do with its result, how is it called?

6

IO.inspect returns its argument so your route function passes the map along to… whatever code is actually calling IO.puts (still not shown :frowning: ) and you get the same error.

Commenting out the whole IO.inspect line will make route return nil, which IO.puts is fine with (it prints a single newline).

4 Likes
7

thanks for all of your comments, the issue were some hidden chars on the file, I created a new file and re-typed the code and the problem went away… need a new keyboard LoL, got to blame it on something…
anywho: alls ok now…

1 Like