>> I would phrase it like this: If you are using, in Prolog code, an atom of length greater than 1, then consider using a list of characters instead, especially

>> I would phrase it like this: If you are using, in Prolog code, an atom of length greater than 1, then consider using a list of characters instead, especially if you want to reason about the data itself within your program. If you use lists of characters throughout, then no conversion to or from longer atoms is needed. That's a very good point and I agree. It reminds a little of how, in formal texts, constants are most often single characters, especially when we just want to refer to some abstract constant in which case we say "c" or "a" or "b" or "α", "β", "γ" and whatever (I do the latter because I have a Greek keyboard and it's easy). But there's a problem with that. In many cases in everyday Prolog programming it makes a lot of sense to represent constants by more than one character. For example, in the perennial family tree motivating example for ILP, you have (FOL) atoms like father(bob,alice) and mother(alice,dhalia) etc. That makes for a very natural notation- "bob" is clearly the name of a person. And then we end up with multi-character atoms that we might need to split or concatenate. Representing such constants as lists of characters would be cumbersome and unnatural- for example father([b,o,b],[a,l,i,c,e]), looks awful, even if it's actually very practical. That's partly what the double-quoted atoms are supposed to achieve, but father("bob", "alice") again introduces unnecessary visual clutter. Perhaps the standard should be for multi-character atoms (or, well, all atoms) to be expanded to lists of atoms by the interpreter by default (without having to double quote) so that you know that writing father(bob,alice) is exactly the same as writing father([b,o,b],[a,l,i,c,e]) and you can then do some list processing, e.g. starting with father(X,Y), then append(X,...) and so on. To be honest, I don't know much about how data are represented in memory in a Prolog engine- I think you know much more about all that, than me. So I can't imagine what the right answer is. A compact, efficient representation of lists of characters sounds useful, for sure.