Macro hygiene

We mentioned that one of the potential dangers when writing macros is that they are unhygienic. This is not specific to Orb - looking up macro hygiene online would give you plenty of reading material on the topic.

So, what is the danger exactly? Let’s look at the following naive implementation of the repeat macro:

mac myRepeat (n body) {
    ret \(range i ,n ,body);
};

Seems good? Now let’s look at this code:

fnc foo () () {
    range i 1 5 {
        myRepeat 3 {
            std.print i;
        };
    };
};

The user is expecting this code to print 111222333444555. What it actually prints is 012012012012012.

If you look at our definition of myRepeat, you will see we used i to name the iterator variable. The user of our macro also used i to name their own variable. Due to how shadowing works, our i is the one being caught in the nested block.

Orb’s macros are unhygienic because they can interfere with the user’s naming scheme. There are multiple types of scenarios where issues like these can show up and catch you off guard. It is advised to read more about macro hygiene to really understand the issue.

One way we can fix myRepeat is to use a unique identifier for the iterator variable and use a different identifier on every invocation.

There is a function in base.orb which does exactly what we need. genId returns a different identifier on every call. So, we should rewrite myRepeat as:

mac myRepeat (n body) {
    sym (myId (genId));
    ret \(range ,myId ,n ,body);
};