Debug Elixir Function Clause errors

function_clause.ex

defmodule FunctionClause do
  @moduledoc """
  Format function clauses using Exception.blame/3
  """

  @doc """
  Given a `module`, `function`, and `args` see
  if that function clause would match or not match.

  This is useful for helping diagnose function
  clause errors when many clauses are generated
  at compile time.

  """
  @spec match(module(), atom(), list(any)) :: :ok | no_return()
  def match(module, function, args) do
    case Exception.blame_mfa(module, function, args) do
      {:ok, kind, clauses} ->
        formatted_clauses(function, kind, clauses, &blame_match/2)

      :error ->
        raise ArgumentError,
              "Function #{inspect(module)}.#{inspect(function)}/#{length(args)} " <>
                "is not known."
    end
  end

  defp formatted_clauses(function, kind, clauses, ast_fun) do
    format_clause_fun = fn {args, guards} ->
      code = Enum.reduce(guards, {function, [], args}, &{:when, [], [&2, &1]})
      "    #{kind} " <> Macro.to_string(code, ast_fun) <> "\n"
    end

    clauses
    |> Enum.map(format_clause_fun)
    |> Enum.join()
    |> IO.puts()
  end

  defp blame_match(%{match?: true, node: node}, _),
    do: Macro.to_string(node)

  defp blame_match(%{match?: false, node: node}, _),
    do: IO.ANSI.red() <> Macro.to_string(node) <> IO.ANSI.reset()

  defp blame_match(_, string), do: string
end

Uglified one-liner (handy for keeping in my IEx history):

defmodule FunctionClause do def match(module, function, args) do case Exception.blame_mfa(module, function, args) do {:ok, kind, clauses} -> formatted_clauses(function, kind, clauses, &blame_match/2); :error -> raise ArgumentError, "Function #{inspect(module)}.#{inspect(function)}/#{length(args)} " <> "is not known."; end; end; defp formatted_clauses(function, kind, clauses, ast_fun) do format_clause_fun = fn {args, guards} -> code = Enum.reduce(guards, {function, [], args}, &{:when, [], [&2, &1]}); " #{kind} " <> Macro.to_string(code, ast_fun) <> "\n"; end; clauses |> Enum.map(format_clause_fun) |> Enum.join() |> IO.puts() end; defp blame_match(%{match?: true, node: node}, _), do: Macro.to_string(node); defp blame_match(%{match?: false, node: node}, _), do: IO.ANSI.red() <> Macro.to_string(node) <> IO.ANSI.reset(); defp blame_match(_, string), do: string; end

Thanks for this btw