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I'm learning the basics of Prolog and I was wondering why the following line prints X = 1 instead of true?

?- X=1,1=X.
X = 1.

--

The first X=1 in my command is an assignment, and the second one will be a check of equality.

false
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Stav Alfi
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    Very simply, that's what Prolog top levels (aka REPLs) do. They collect all free variables in the query and show their values upon each success. This is probably an arbitrary choice. Some implementation could choose to never show anything unless explicitly requested in the query. Some don't show values for variables starting with the underscore, like `_X`. Probably all won't ever show any `_` (i.e. anonymous variable)'s value. And welcome to Prolog! :) – Will Ness Mar 06 '20 at 15:14

2 Answers2

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There are no assignments or equality tests in your query, only unification of terms. The query succeeds by unifying the variable X with 1 and that's what the top-level reports: it tells which variable bindings makes the query true.

After the first goal in the conjunction, X = 1, succeeds, the second goal is the unification 1 = 1, which trivially succeeds.

P.S. Also note that Prolog systems differ in the way they report successful queries. Some print true, others print yes (the traditional way that successful queries are reported).

Paulo Moura
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  • Thanks. I am little bit confused. as I understand from you `1=1.` is also a unification. So why `1=1.` prints `true`, but `X=1,1=X.` prints `X=1`? – Stav Alfi Mar 06 '20 at 14:11
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    The `1 = 1` unification succeeds without binding any variables. Thus, the top-level interpreter will simply print `true` in this case (or `yes` depending on the used Prolog system). – Paulo Moura Mar 06 '20 at 14:21
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When the answer is true and a value is bound to variable at the top level, the value of the variable is displayed, which implies the result was true.

Here are some examples.

test_01 :-
    X = 1,
    X = 1.

test_02 :-
    X = 1,
    X = 2.

test_03(X) :-
    X = 1,
    X = 1.

test_04(X) :-
    X = 1,
    X = 2.

and when the examples are run from the top level using SWI-Prolog

?- test_01.
true.

?- test_02.
false.

?- test_03(X).
X = 1.

?- test_04(X).
false.

Here are some examples that are done only in the top level

?- X=1.
X = 1.

?- 1=1.
true.

?- 1=0.
false.

?- 1==0.
false.

The first X=1 in my command is an assignment, and the second one will be a check of equality.

X=1 is not an assignment it is a unification of the integer 1 to the variable X. The second X=1 is not a check of the equality, it is another unification of X to 1, but since X is bound to 1 by this time, it is really a different unification.

To do equality checking in Prolog use ==, e.g.

?- 1 == 1.
true.

?- 1 == 2.
false.

Also , is the logical and, so if 

?- X = 1.
X = 1.

then 1 is bound to X and is true and similar for the second line in your question.

However the code has to be also viewed as

?- true,true.
true.

as opposed to

?- true,false.
false.

While ; is logical or

?- true;true.
true ;
true.

?- true;false.
true ;
false.

?- true;false;true.
true ;
true.

?- false;true.
true.

?-  false;false.
false.

Notice that the first 3 answers have 2 results, but the last two answers have 1 result.

Guy Coder
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  • Thanks, can you explain why `Notice that the first 3 answers have 2 results, but the last two answers have 1 result.` ? – Stav Alfi Mar 06 '20 at 16:28
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    If you run more examples like this you will find out that *false* results are not returned unless it is the last result. – Guy Coder Mar 06 '20 at 16:30