Adding error handling

lib/joken/error.ex

@@ -42,6 +42,13 @@ defmodule Joken.Error do
     Joken.Config.default_claims/1.
     """
 
+
+  def message(%__MODULE__{reason: :algorithm_needs_key}),
+    do: """
+    A map was expected for the key parameter in the signer creation. 
+    This is mandatory for: #{inspect(Signer.map_key_algorithms())}.
+    """
+
   def message(%__MODULE__{reason: :unrecognized_algorithm}),
     do: """
     Couldn't recognize the signer algorithm.

lib/joken/signer.ex

@@ -40,6 +40,13 @@ defmodule Joken.Signer do
   @spec algorithms() :: [binary()]
   def algorithms, do: @algorithms
 
+
+  @doc """
+  Map key algorithms.
+  """
+  @spec map_key_algorithms() :: [binary()]
+  def map_key_algorithms, do: @map_key_algorithms
+
   @doc """
   Creates a new Joken.Signer struct. Can accept either a binary for HS*** algorithms
   or a map with arguments for the other kinds of keys. Also, accepts an optional map
@@ -90,6 +97,8 @@ defmodule Joken.Signer do
     )
   end
 
+  def create(alg, _key, _headers) when alg in @map_key_algorithms, do: raise(Joken.Error, :algorithm_needs_key)
+
   def create(_, _, _), do: raise(Joken.Error, :unrecognized_algorithm)
 
   defp raw_create(alg, jws, jwk) do

test/joken_signer_test.exs

@@ -54,6 +54,12 @@ defmodule Joken.Signer.Test do
            } = signer
   end
 
+  test "raise with invalid parameter" do
+    assert_raise Error, Error.message(%Error{reason: :algorithm_needs_key}), fn ->
+      Signer.create("RS256", "Not a map")
+    end
+  end
+
   test "raise with invalid algorithm" do
     assert_raise Error, Error.message(%Error{reason: :unrecognized_algorithm}), fn ->
       Signer.create("any algorithm", %{})