[tom_mellior]

I think -> is newer, but it's in ISO Prolog, which came out in 1995. So it's rather likely that the OP learned Prolog at a time when -> existed but teaching resources had not caught up. Not sure if that's the case even now.

[talideon]

I learned Prolog back in 1997, so it may have existed in the implementations I used back then. Must see if I can find an implementation of HU Prolog from back then. I also used SWI Prolog from back then, and I'd expect that was more up to date.

Edit: mention SWI Prolog.