How does = operator works in Prolog

Sorry if this is a newbie question, but recently I was trying to compare an string and I used this (not exactly :P):

some_fact('Yes').
some_fact('No').
some_rule(X):- some_fact(X), (X =:= 'Yes' -> writeln("ISS YES") ; writeln("No")).

Error: Arithmetic: `'Yes'' is not a function

After that, I Googled and saw that Strings are compared with = and \=

But if I write: X = 5 I'm assigning the value 5 to X, well I don't know if the word is assign, cause the assigment operator is is. Right?

Just in case, I don't need to fix the code, I want understand what's happening. Thanks

1 Answer

I think there is a lot of confusion here and most of it would be remedied by going through a book, but let me try and clear a few things up for you right now.

1. 'Yes' is an atom, not a string. SWI-Prolog has actual strings, but most Prolog implementations do not, they use atoms instead. The nice thing about atoms is that if they are lower case and do not contain spaces there is no need for quotes. The quotes are needed to tell Prolog "this is not a variable" and resolve the syntactic ambiguity of this and that.

2. Lacking strings, there is no operator for string comparison.

3. = is the unification operator. Unification is a big topic—not something that is easily summarized in a question, but as an approximation you can think of it as a bi-directional pattern matching. So, it will do the job for what you probably need string comparisons for, but it is the real engine of computation in Prolog and is happening behind the scenes in lots of ways.

4. Prolog does not have assignment. True, you can give a variable a value. But you cannot change that value later; X = X + 1 is meaningless in mathematics and it's meaningless in Prolog too. In general, you will be working recursively, so you will simply create a new variable when something like this needs to occur. It will make more sense as you get further in reading about Prolog and writing your first programs. Please check out a tutorial!

5. is/2 resolves arithmetic expressions. If you have X = 2+3, Prolog will reply with X = 2+3. Only X is 2+3 will cause Prolog to report X=5. Arithmetic just isn't a huge part of classic Prolog usage; these days, people will quickly suggest you check out CLPFD, which enables you to do more interesting things like 15 #= X + Y and producing bindings that add up to 15. Standard Prolog cannot "work backwards" like this. But for a complete beginner it probably suffices to say that arithmetic works differently than you expect it to, and differently than the rest of Prolog unless you use CLPFD.

6. =:= is an arithmetic equality operator. You use this to answer questions like 6 + 1 =:= 5 + 2. This is a really special-purpose tool that I personally have never really needed to use.