Hello World!

Okay, so I have been thinking for some time to start a blog, and write about what bothers me, what keeps me up at nights, what keeps me sleeping at days... :)

I guess I'll start with the title; append/3 is the first Prolog predicate I actually understood how it works. And believe me when I say "there became light".


append([],L,L).
append([X|L1],L2,[X|L3]) :-
    append(L1,L2,L3).




append/3 appends its second argument in the end of its first, and returns the result in the third argument. But not just this. Let's first say we want to concatenate two lists. So, we'll call append/3 with arguments 1 and 2 bound, and 3 unbound:

| ?- append([1,2,3],[4,5,6],L).

L = [1,2,3,4,5,6];

no

Cool, so it does what we want it to do. How about something...kinky? :) Say we want to call append/3 with arguments 1 and 2 unbound (i.e. replaced by variables), and argument 3 bound. Then, magic kicks in, and argument 3 is split into all the possible lists that when concatenated together give that as a result:



| ?- append(X,Y,[1,2,3]).

X = []
Y = [1,2,3];

X = [1]
Y = [2,3];

X = [1,2]
Y = [3];

X = [1,2,3]
Y = [];

no



That "magic" is the power of Prolog, and computer science people call it "non-determinism". It means that one Prolog query can have multiple answers. And that's just the beginning of it :)




Prolog uses assign-once variables; that means that once a variable gets a value, that value can't change throughout the execution of a program. You'd ask yourself why on earth can something like this be useful. And I'll just reply with a quote: "In Logic, variables are also assign-once; you can't change the value of a logic variable during the evaluation of a logic formula. And since Logic has been around long before Programming Languages were, you can see who messed-up the meaning of 'variables'".