prolog - Order of unification in lists when using pipe(|)

Question

I am having trouble trying to figure out in what order unification is done. I have the following query:

[X, like | Z] = [I, Y, Prolog, language].

This gives me the following result:

X = I,
Z = [Prolog, language],
Y = like.

However, what I was expecting is:

X = I,
Y = like,
Z = [Prolog, language].

Is there any specific order in which Prolog unifies terms?

EDIT: I have a feeling that Prolog is giving higher priority to unification of an atom with a variable over that of a variable with an atom. Is that so?

question from:https://stackoverflow.com/questions/65873598/order-of-unification-in-lists-when-using-pipe

Answer 1

There is no order. Here is the mapping that is performed:

[I,  Y   , Prolog, language].
 |   |     +--------------+
 |   |           |
[X, like |       Z] 

if that mapping is possible (i.e. if the variables I ,Y, X, Z are bound to terms that allow unification to succeed)

In this case, assuming that all variables are unbound, then:

• Variables I and X will become "the same variable" (more clearly, the variable name I and X will designate the same empty storage cell)

• Variable Y will become the atom like' (more clearly, the storage cell designated by variable name Ywill be set tolike`)

• Variable Z takes up the "rest (or suffix) of the list" (everything behind the |) and will become the complex term `[Prolog, language]'.

• Variable Prolog is not changed and stays what it was.