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Learning Prolog: Lab Session #2

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Apparently we are supposed to complete the second and third set of lab instructions before the next lab. This post is about what I've learnt from the second lab session, and my thoughts along the way.

Now that I've played around with Prolog for a bit, I've discovered that it isn't really as bad as I thought it was to start with, although the inbuilt editor is absolutely terrible (and there's no real replacement :/).

Last time I learnt that Prolog is rather like a detective in that you tell it about the world, and it can then answer simple questions based on what it's learnt. It's also capable of working things out that you don't explicitly tell it, for example that cats don't like bones - they like milk instead.

Today's lab session started off with some simple Maths. Prolog, apparently, isn't particularly good at Maths - and I'm inclined to agree. Maths in Prolog is done through the use of the symbol comparison system. For example, you can ask Prolog is 2 is equal to itself:

?- 2 = 2.
true.

Prolog replies that yes, 2 is equal to itself. This is relieving, as something would be very wrong otherwise. You can ask Prolog whether other things are equal to each other:

?- two = two.
true.
?- carrots = carrots.
true.

This is mildly interesting, but not really useful. Let's ask it something a bit harder.

?- 4 = 2 + 2.
false.

Erm right. That's not the answer we were expecting...! The reason for this is that Prolog sees the above in terms of the literal symbols that we gave it. So Prolog sees this instead:

Is four equal to "two plus two"?

The equals operator in Prolog actually means unify (are these things the same?), and Prolog is a bit stupid and takes this more literally than we intended. Thankfully, there's another operator we can use:

?- 4 is 2 + 2.
true.

That's much better. The is operator can be used to check if two things are equal as well, but the difference here is that Prolog will perform any mathematical operations that it comes across on the right hand side. In addition, it can be used to set variables (more on that later). The is operator supports brackets, too, allowing you to do this:

?- 40 is ((2 * 10) / 4) * 8.
true.

Variables

Variables in Prolog always start with a capital letter, and are useful when you want to ask Prolog questions like "What does a cat like?" or "What is purple?". Here's an example knowledge base:

purple(flower).
purple(elephant).
animal(elephant).
animal(cat).

Given the above information, we know we can ask Prolog questions like these:

?- purple(elephant).
true.
?- purple(cat).
false.

Those are nice, but rather obvious. Let's step it up a bit, and ask it what is purple:

?- purple(What).
What = flower .

Note that you have to hit enter after entering the command to confirm that the answer is correct. In the above it has correctly identified that a flower is purple and put this information into the variable What. Let's see if we can retrieve the contents of the variable later on:

?- What.
% ... 1,000,000 ............ 10,000,000 years later
% 
%       >> 42 << (last release gives the question)

What's going on here?! This is certainly a rather odd response. The problem here is that variables in Prolog are scoped to the command that's executing. This seems rather useless for now, but at least we use them to ask Prolog a different sort of question. This also works with multiple predicate parameters:

food(cheese, dairy).
food(milk, dairy).
food(cabbage, vegetable).
food(orange, fruit).

Given the above knowledge, we ask things like:

?- food(What, vegetable).
What = cabbage.
?- food(What, dairy).
What = cheese .

Prolog correctly identifies that a cabbage is a vegetable, and that cheese is a dairy product. Note that it picked cheese over milk, as cheese is the first thing it came across. I have a feeling this might be important later on.

That concludes this post on Prolog. If you found it useful, please comment below! I don't run any sort of analytics on here at the moment, so I can't who's reading (or not). If you found a mistake (which is likely) or just want to say hello, please also leave a comment below.

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