When you're using pattern matching to also create variables, it's just really confusing to be able to create variables with both sides of the expression. It can

When you're using pattern matching to also create variables, it's just really confusing to be able to create variables with both sides of the expression. It can get confusing enough as it is. Haskell has some extensions to pattern matching that AIUI are generally considered just too "powerful" to generally use, as it makes it very difficult to read, for instance. Plus to a first approximation you can always just swap the binding from the right to the left. A deeper analysis may reveal there's some case you can't do that, there's a lot to pattern matching and I'm not going to try to sit here and construct a case where you can't, but certainly the most common cases (by far) would just require a simple swap, so it's not an onerous requirement. And if you did construct a case where you can't, you could make a very solid software engineering case that you just shouldn't do that and should do it it two parts. To the extent this doesn't apply to prolog, it would be when prolog is functioning directly as a logic language rather than a programming language, and to be honest I'd still advise against a prolog programmer binding variables on both sides of an expression unless they're forced to for some reason just because if nothing else, you may have to understand your own code a week later. Note this specifically applies to binding variables; I'm not bothered by two already-bound variables being matched against each other, that's just prolog functioning as designed.