What's the difference between $?CLASS and ::?CLASS

Question

The Raku docs describe ::?CLASS as a compile-time variable that answers "Which class am I in?". Then, a couple of paragraphs later, it mentions $?CLASS, and says that it answers "Which class am I in? (as variable)".

What's the difference between these two variables? They often seem to contain the same value – do they always? Or are there differences I haven't observed?

Answer 1

These two forms of compile-time variables are different syntactically.

The syntactic form ::?foo may be used as, and will be evaluated as:

• A term (just like a variable such as $?foo).

• A type constraint (unlike a variable such as $?foo).