Can I really change standard operator priorities in ALGOL 68?

Question

In Algol 68, I can declare the priority (precedence) of an operator-symbol:

    prio @ = 5;

(Higher number means higher precedence).

I can apparently redefine the priorities of built-in operators:

    begin
    prio + = 7, x = 6;
    print(2+3x4)
    end

This should result in printing the value 20, rather than the 14 that a standard parse would give.

Did actual implementations actually behave that way? With a quick scan, I see nothing in either the Informal Introduction to ALGOL 68 nor the ALGOL 68-R Users Guide to say it wouldn't. Still, I find that mutability to be a strange feature.

(I should try it and see but I don't have an ALGOL 68 system at my fingertips right now).

Answer 1

Yes you can, and it is also scoped (as you would expect with an orthogonal language like Algol 68). On Algol 68 Genie you get the following result:

C:\Users\Brian>a68g SODave.a68
        +20        +14

C:\Users\Brian>type SODave.a68
BEGIN
  BEGIN
    PRIO + = 7, * = 6;
    print(2+3*4)
  END;
print(2+3*4)
END

C:\Users\Brian>

Answer 2

This is just an answer to @Will Hartung's query about why priority changes were added. Say there is a list with two routines

ref List ListAdd(ref List ll, int item)
ref List ListRemove(ref List ll, int item)

We can code as

ListAdd(lll, item1);
ListRemove(lll, item2);

Instead of doing that, we can define two operators

op + = (ref List ll, int ii) ref List 
op - = (ref List ll, int ii) ref List

So we can do

lll := lll + 3;
lll := lll - 4;

What if we wanted to combine the operations?

lll := lll + 3 - 4;

Since + and - are the same priority, the order is indeterminate. It could be

lll := (lll + 3) - 4

or

lll := lll + (3 - 4);

If I remember correctly, the default priority of + and - is 6. There are obscure things that you remember even though you haven't used them for 40+ years. If we want the first case, we could change the priority of + to be 7 so that the + always happens first.

Alternatively, if you do no wish to redefine priorities, do it as two separate statements or just use parenthesis.