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rakucompile-timecompile-time-constantrakudo

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

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?

Solution

  • 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).