Comparing

The six comparisons, and the answer to the question left hanging about the = sign.

Comparing two values gives a bool — true or false. You have already met one without paying it much attention: the registered variable in the “Showing a value” sub-section.

@entry(start)
object MyProgram {
    start() {
        var age := 42;

        Console.print(text: $"is age 42? {age = 42}");
        Console.print(text: $"does age differ from 42? {age != 42}");
        Console.print(text: $"is age over 18? {age > 18}");
        Console.print(text: $"has age reached 18? {age >= 18}");
    }
}

Four questions, four answers.

= compares, and never assigns

Here is the answer promised in 2.1. The = sign is reserved for equality: age = 42 does not put 42 into age, it asks whether age is 42, and gives back true or false.

That is why assigning is written :=. Each gesture has its own sign, and the equals sign means what it has meant in mathematics all along.

So there is no == in this language. Where C, Java or Python write = to assign and == to compare, Archon has no need to double the sign to tell the two apart — they do not look alike in the first place.

The gain is not cosmetic. Confusing = and == is one of the oldest and costliest mistakes of the C family: if (x = 5) is perfectly valid code there, assigning where it meant to compare, and going through without a word. Here the mistake has no shape: you cannot write one believing you wrote the other.

The six

!= reads “differs from”. It is the only place in the language where the exclamation mark appears: it is reserved for this use, and negation itself is written out in full — we come to it in 3.3.

Texts compare too

= is not only for numbers. Two texts are compared by their contents:

var word := "Alexandria";
Console.print(text: $"{word = "Alexandria"}");   // true

Two identical texts are equal, even if they were written at two different places in the program.

Your turn

• Change 42 to another value in the four questions, and predict each answer before running.
• Compare two texts that differ only by a capital — "simon" = "Simon". The comparison is exact, it forgives nothing.
• Write age = 43; alone on a line, as a statement. The compiler refuses — a comparison whose result is thrown away does nothing — and names in passing the sign you may have meant.