title: Free monad interpreter with side effect only
body: <p>I need to write free monad interpreter which only perform effectfull actions but does not return any result. For free applicative I used interpreter with <code>MyAction ~&gt; Const[Unit, ?]</code> signature and called it using <code>foldMap</code> function. However Const does not have monad instance. <code>Id</code> does not suit because it requires to return actual value (evaluate embedded language). Should I write my own monad instance, similar to Id but without underlying value or there is a more convient way to create interpreter which does not produce any values?</p>