Unifiable Prolog Terms

Question

Are the two terms [a,b,c,d,e] and [[X] | [b,c,d,e]] unifiable? I need confirmation I think they are not because you cannot get [X] = a in any way?

Answer 1

I think you need to unify with [X | [b,c,d,e]], not [[X] | [b,c,d,e]], i.e. you can get X=a, not [X]=a.

Answer 2

Your assumption is correct: [X], that we can rewrite as .(X,[]) cannot be unified with the atom a.

We can rewrite [X|[b,c,d,e]] as .(_G231,.(b,.(c,.(d,.(e,[]))))), i.e. [X,a,b,c,d,e].

You can inspect see the shape without syntax sugar using display/1.

Answer 3

if X needs to be enclosed in a list for some reason, you will not be able to have this unify (as described by chac). but you can use the append predicate to get a match:

?- append([X], [b,c,d,e], [a,b,c,d,e]).
X = a