Understanding pragmas

Question

I have a few related questions about pragmas. What got me started on this line of questions was trying to determine whether it's possible to disable some warnings without going all the way to no worries (I'd still like to worry, at least a little bit!). And I'm still interested in the answer to that specific question.

But thinking about that issue made me realize that I don't really understand how pragmas work. It's clear that at least some pragmas take arguments (e.g., use isms<Perl5>). But they don't seem to be functions. Where do they fit into the overall MOP? Are they sort of like Traits? Or packages? Is there any way to introspect over them? See what pragmas are currently in effect?

Are pragmas built into the language, or are they something that users can add? When writing a library, I'd love to have some errors/warnings that users can optionally disable with a pragma – is that possible, or are they restricted to use in the compiler? If I can create my pragmas, is there a practical difference between setting something with a pragma versus with a dynamic variable, aside from the cleaner look of a pragma? For that matter, how do we decide what language features should be set with a pragma versus a variable (e.g., why is $*TOLERANCE not a pragma)?

Basically, I'd be interested in any info about pragmas that you could offer or point me towards – though my specific question is still whether I can selectively turn off certain warnings.

Answer 1

Currently, pragmas are hard-coded in the handling of the use statement. They usually either set some flag in a hash that is associated with the lexical scope of the moment, or change the setting of a dynamic variable in the grammar.

Since use is a compile time construct, you can only use compile time constructs to get at them (currently) (so you'd need BEGIN if it is not part of a use).

I have been in favour of decoupling use from pragma's in the past, as I see them as mostly a holdover from the Perl roots of Raku.

All of this will be changed in the RakuAST branch. I'm not sure what Jonathan Worthington has in mind regarding pragmas in the RakuAST context. For one thing, I think we should be able to "export" a pragma to the scope of a use statement.