如何将比较结果存储在 Prolog 中并稍后使用？

Question

Say I want to write an (admittedly unnecessary, but it's an example) predicate that compares two values, and returns the result as a variable that can be referenced, something like this:

compare(Value1, Value2, Result) :- 
    Result is Value1 > Value2.

But in Swish I get an error:

src:2: Syntax error: Operator priority clash
compare/3: Domain error: `order' expected, found `'5''

After I have Result , how would I then use its value in another predicate? Would it be possible to say:

compare(5, 2, Result),
Result.

Or am I completely misunderstanding the philosophy of Prolog?

Answer 1

Superficially, it's a problem of operators' precedence: this compiles

compare(Value1, Value2, R) :- R is (Value1 > Value2).

but it doesn't run:

?- compare(1,3,X).
ERROR: compare/3: Type error: `atom' expected, found `1' (an integer)

You're going to clash with a builtin, see compare /3. Better to avoid, although in SWI-Prolog redefine_system_predicate /1 directive could help.

is /2 is a predicate implementing a small (?) functional sublanguage, evaluating right side arithmetic expression, while > /2 implements the comparison after evaluating both sides. What I mean:

..., A > B, ...

it's either true or false, doesn't yields a number...

Maybe you want

my_compare(A,B,C) :- A > B -> C = 1 ; A < B -> C = -1 ; C = 0.

edit :

I think I should illustrate how to store the comparison value: just instance the result and call it later.

my_compare(A,B,C) :- A > B -> C = true ; C = false.

then

?- my_compare(1,3,R), (R -> writeln('comparison succeeded') ; writeln('comparison failed')).