[ruby-core:118675] [Ruby master Feature#19979] Allow methods to declare that they don't accept a block via `&nil`

Issue #19979 has been updated by matz (Yukihiro Matsumoto).


This proposal is suspended due to the Syntax Moratorium. Since this might cause "no block policing" in the community, let me think while (at least til 3.5).

Matz.


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Feature #19979: Allow methods to declare that they don't accept a block via `&nil`
https://bugs.ruby-lang.org/issues/19979#change-109207

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Open
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## Abstract

This feature proposes new syntax to allow methods to explicitly declare that they don't accept blocks, and makes passing of a block to such methods an error.

## Background

In #15554, it was proposed to automatically detect methods that do not use the block passed to them, and to error if a block was passed to such methods. As far as I can tell, it was later on closed since #10499 solved a large part of the problem.

That proposal has, as part of [a dev meeting discussion](https://github.com/ruby/dev-meeting-log/blob/b4357853c03dfe71b6eab320d5642d463854f50f/2019/DevMeeting-2019-01-10.md?plain=1#L110-L120), a proposal from @matz to allow methods to use `&nil` to explicitly declare that they don't accept a block. At the time, the proposal was trying to solve a bigger problem, so this sub-proposal was never considered seriously. However, notes in the proposal say:
> It is explicit, but it is tough to add this `&nil` parameter declaration to all of methods (do you want to add it to `def []=(i, e, &nil)`?). (I agree `&nil` is valuable on some situations)

This proposal extracts that sub-proposal to make this a new language feature.

## Proposal

In Ruby, it is always valid for the caller to pass a block to a method call, even if the callee is not expecting a block to be passed. This leads to subtle user errors, where the author of some code assumes a method call uses a block, but the block passed to the method call is silently ignored.

The proposal is to introduce `&nil` at method declaration sites to mean "This method does not accept a block". This is symmetric to the ability to pass `&nil` at call sites to mean "I am not passing a block to this method call", which is sometimes useful when making `super` calls (since blocks are always implicitly passed).

Explicitly, the proposal is to make the following behaviour be a part of Ruby:
```ruby
def find(item = nil, &nil)
  # some implementation that doesn't call `yield` or `block_given?`
end

find { |i| i == 42 }
# => ArgumentError: passing block to the method `find' that does not accept a block.
```

## Implementation

I assume the implementation would be a grammar change to make `&nil` valid at method declaration sites, as well as raising an `ArgumentError` for methods that are called with a block but are declared with `&nil`.

## Evaluation

Since I don't have an implementation, I can't make a proper evaluation of the feature proposal. However, I would expect the language changes to be minimal with no runtime costs for methods that don't use the `&nil` syntax.

## Discussion

This proposal has much smaller scope than #15554 so that the Ruby language can start giving library authors the ability to explicitly mark their methods as not accepting a block. This is fully backward compatible, since it is an opt-in behaviour and not an opt-out one.

Future directions after this feature proposal could be a way to signal to the VM that any method in a file that doesn't explicitly use `yield`/`block_given?` or explicitly declared a block parameter should be treated as not accepting a block. This can be done via some kind of pragma similar to `frozen_string_literal`, or through other means. However, such future directions are beyond the scope of this proposal.

## Summary

Adding the ability for methods to declare that they don't accept a block will make writing code against such methods safer and more resilient, and will prevent silently ignored behaviour that is often hard to catch or troubleshoot.



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