PROLOG: process of inference, why returns false

Question

I have a definition of conc:

conc([], L2, L2).
conc([X1|R1], L2, [X1|RN]) :-
   conc(R1, L2, RN).

I don't understand why conc([X | green], Y, [red, green, blue]). returns false rather than

X = [red],
Y = [blue]

What is the process of inference here?

Answer 1

Disclaimer: I don't know Prolog. The rest of this answer is an edumacated guess.

Your proposed solution of X = [red] doesn't make sense because that would make X a one-element list. Let's assume

X = red

instead.

That would give us

conc([red | green], [blue], [red, green, blue]).

With the second equation of conc that turns into

conc(R1, L2, RN).
% with:
% X1 = red
% R1 = green
% L2 = [blue]
% [X1|RN] = [red, green, blue]
% i.e. X1 = red
%      RN = [green, blue]

I.e.

conc(green, [blue], [green, blue]).

And now we're stuck because none of your conc rules applies to green.

The problem is [X | green] because green is not the tail of a list.

Did you mean [X, green] instead?