[Harrison Pratt]

Thomas,
I was wasting some time noodling around in Google Gemini and this came up. I seem to remember that you indicated that "in" was preferable for Visual Prolog's current implementation of these predicates. Of course, for the small scale of problems I deal with the difference is probably minimal.

Do you have any comments on the Gemini response below:

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**Member vs. In: A Performance Comparison**

In Prolog, the `member` predicate and the `in` operator are both used to check if an element is present in a list. However, `member` is generally more efficient than `in` for several reasons:

1. **Tail Recursion:** The `member` predicate is often implemented using tail recursion, which allows the Prolog interpreter to optimize the recursion by reusing the same stack frame. This can significantly improve performance, especially for large lists.
2. **Built-in Optimization:** Prolog interpreters often have built-in optimizations for the `member` predicate, such as indexing or other techniques, that can further improve its performance.
3. **Direct Access:** The `member` predicate can sometimes directly access elements in the list based on its implementation, which can be faster than the `in` operator, which might perform a linear search.

**In Summary**

While both `member` and `in` can be used to check for list membership in Prolog, `member` is generally more efficient due to its tail-recursive nature, built-in optimizations, and potential for direct access. For most use cases, it's recommended to use `member` for better performance.

[Thomas Linder Puls]

Well that text is aimed at some traditional Prolog systems. It is definitely not true for Visual Prolog, and I also doubt very much that it is true for any other Prolog system. Why should you make special effort for optimizing user code and not apply the same optimizations to built in features. In facts, I could easilier see the opposite: that some built in feature has received special attention.

Anyway in Visual Prolog the two are exactly the same - exactly.