Use a function that prints then returns its parameter:

  list(1, 2, 3, 4).reduce{print(_) + _} == 10

Use it if you or your language hasn't defined such a function:

  list(1, 2, 3, 4).reduce{it:= _; println(it); it + _} == 10

In fact, any name for it will do.

Yeah, that's the tradeoff–gain the ability to work with multiple parameters but lose the ability to reuse a single one.