gistfile1.ex

defmacro defguard({name, _, args}, [do: body]) do

  quote bind_quoted: [args: args, body: body, name: name] do
    defmacro unquote(name)(unquote_splicing(args)) do
      if Macro.Env.in_guard?(__CALLER__) do
        quote do
          unquote(body)
        end
      else
        eager_bindings = Enum.map args, fn({name, _, _}) ->
          {:=, [], [{name, [], nil}, name]}
        end
        quote bind_quoted: [eager_bindings: eager_bindings, body: body], unquote: true, do: unquote(body)
      end
    end
  end
end

defguard foo(bar), do: bar == "bar"
# => (CompileError) undefined function bar/0

defmacro defguard(fun, [do: body]) do
  {name, args} = Macro.decompose_call(fun)
  quote bind_quoted: [args: args, body: body, name: name] do
    defmacro unquote(name)(unquote_splicing(args)) do
      if Macro.Env.in_guard?(__CALLER__) do
        quote do: unquote(body)
      else
        quote bind_quoted: binding, unquote: true, do: unquote(body)
      end
    end
  end
end