pre-RFC, 'gather macro'

dobkeratops · July 4, 2017, 9:20pm

dobkeratops/rfcs/blob/gather_macro/text/0000-gather_macro.md

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# Summary

generalize the ability to gather invocations (exhibited by #[test]) , easily accessible in a proposed macro ```gather!()``` and attribute ```#[gather_into(<macro_name>)]```
see also forum post https://users.rust-lang.org/t/gather-macro-invocations-e-g-class-factory-roll-one-big-enum/11674

# Motivation

Potentially useful for rolling 'class factories', 'component systems', bindings to scripting languages/UIs; also a similar use case is already demonstrated in the inbuilt unit tests, where ```#[test]``` gathers many functions and calls them from a generated test function. Macros accessible from the language out of the box will see more use than compiler plugins.

Rust offers a choice between 'sorting by type', and 'sorting by function' (e.g. polymorphism through traits, or matching on enum variants). A user could wrap seperate types and functions in a macro that can be rolled either way. (e.g. many structs could be ```gather!```ed into a tuple struct,or many functions could be ```gather!```ed into a single match body.)

Some use cases are already possible with nested repeat features , but they require items to be declared in one location (e.g. a project may prefer to have 'a source file per entity', rather than one source file defining all the entities)

## Possible examples of applicability

###'high order macro'

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any thoughts on this idea, a ‘gather’ feature in the macro system, allowing macros to collect information from elsewhere, or allowing pieces of recursive macros to be collected

any other ideas on how to achieve the goals here? I know the existing macro system is very capable.

matklad · July 5, 2017, 6:04am

Cargo does something similar for subcommands with a high order macro: https://github.com/rust-lang/cargo/blob/eb6cf012a6cc23c9c89c4009564de9fccc38b9cb/src/bin/cargo.rs#L101-L134

You still are required to explicitelly list all the entries in one place, but then you can use arbitrary macros to process them.

dobkeratops · July 6, 2017, 3:20am

reminiscent of the x-macros trick in C, but obviously a lot more sane here.

So basically this is an example of a case where a ‘gather macro’ might help (generating that list). i guess in that case you could probably avoid the need to ‘maintain the list’ by embedding all the original code in the macro, but gathering from multiple source files would be saner for project organization

system · March 25, 2019, 8:28am

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pre-RFC, 'gather macro'

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