[Moving to haskell-cafe] Hi,

I want to know if it exists a null instruction (a instruction that do anything

Not really, since instructions don't "do anything" anyway - they only compute values. If you are in the IO monad then "return ()" is the do nothing. In the function world, "id" is the do nothing. In recursive list processing, then something like [] is probably the "base case" in a recursive function, so stops the recursivity. If you give a more concrete example, then maybe we can give you a better answer. Thanks Neil