prolog

Syntax error: Operator expected in Prolog

I am writing a small interpreter in SWI Prolog and run into the following problem: I need to write a predicate that will determine if logical expression is true. I already have a predicate

evaluate_arithmetic_expression(+Expr, +State, -Value)

that evaluated arithmetic expression in a given state. Now I need a predicate for logical expressions:

is_logical_expression(+Expr, +State)

The following clause appears to work just fine:

is_logical_expression_true(Expr1 = Expr2, State) :-
  evaluate_arithmetic_expression(Expr1, State, Value1),
  evaluate_arithmetic_expression(Expr2, State, Value2),
  Value1 =:= Value2.

But this won't compile:

is_logical_expression_true(Expr1 <> Expr2, State) :-
  evaluate_arithmetic_expression(Expr1, State, Value1),
  evaluate_arithmetic_expression(Expr2, State, Value2),
  Value1 =\= Value2.

And I get error:

Syntax error: Operator expected

It looks like <> operator binds harder than anything else, but I don't think <> is even an operator in SWI. How can I fix this?

Solution

  • User-defined operators must be declared by using the ISO predicate op/3:

    :- op(700, xfx, (<>)).