Issue #19979 has been updated by rubyFeedback (robert heiler).
Is this a frequent error?
I think the only times I saw it is due to confusion of
the priority
of do/end vs {/} (which needs to be learned separately anyway), and
for some poorly-designed open methods which don't take a block.
Personally I never saw such an error and can not recall having made
such an error either. I also never had a need to prevent passing of
blocks, but I also agree that there may be situations where one may
specifically want to disable that, even though I personally never had
such a use case.
I think the only times when I had issues with calling parent methods,
and it confused me, was when I used super() versus super without the
(). Since then I, oddly enough, transitioned towards using super()
as default (unless I need the behaviour of super without () specifically).
If we reason that ruby is to be a flexible language, then I think we
can reason that ruby should have a way to disallow passing a block,
just as you, as a ruby developer, can decide on how many arguments
a method should accept, including keyword arguments (which, by the
way, also confused me when I first saw them; I also dislike when a
third party API demands of me to use keyword arguments - I always
felt that an option hash is less restrictive and in some ways
equivalent.)
Note that personally I am not a huge fan of the &nil, but I also
don't want to be too discouraging. One concern I have is that
it may still confuse users, when they see &block and then &nil
and then &urmom ... not sure I am a huge fan of that. I kind
of default to &block - somehow to me this is the most sensible
name for a block variable to be had. If others use &nil, in their
code base - well, they can do what they want to in their code
base. I only focus on trying to maintain my own ruby code and
hope that there is some sanity in what I maintain. :)
But, the TL;DR, I agree that it does not seem to be too common,
at the least I can not recall having seen it before. I don't
know all the ruby code out there, though.
I think even if it was accepted it would be very
unlikely
for gems to use &nil, first they would need to require
Ruby 3.3+ and second it's extra noise in the source code
with almost no benefit for the gem.
So I think in practice this would not achieve much.
I kind of agree with that, but I suppose this may be a long
term suggestion and change, so you could re-evaluate in,
say +3 years. It may not be too overly common though; I can
not think of any real use case I have where I specifically
need to prevent passing of a block. To me I always treated
block variables as a nice, additional argument to allow for
more flexibility, rather than as a basic-must-have that
is the main driving factor for designing APIs, libraries and
use cases.
I do not think that "only on ruby 3.3" is a necessary exclusive
criterium, though. For instance, I try to keep my code always
up-to-date with the latest ruby xmas release. Sometimes I may
try to maintain backwards compatibility, but it is never my
primary focus. I always default with MRI these days; it simply
is easier to do so. (I understand that some implementations lag
behind, e. g. jruby - I think they are still not at 3.2 or so
if I recall headius' summary-issue-tracker for it.)
For methods which sound like they might take a block
but
don't, the raise ArgumentError, ... above seems a good
existing solution.
Understandable as well, but I would like to point out that
raising an error may not necessarily always be that useful
or necessary. For instance, I tend to not care too much about
block variables, so I rarely ever raise an error about it.
At best I may give a notice to the user, but that's about it
for block variables usually.
It's also a bit similar to **nil, which is almost
never used.
Agreed. I actually also never used **nil so far.
zverok wrote:
So I am also not sure where the currently proposed
feature would help.
Agreed, but matz also discussed this before, so there may be a use
case, even if it may be very small.
zverok wrote:
Also, as a negative consequence, I can imagine a
linter-enforced
rule to "always add this declaration when the method doesn't
need/accept block," which will make a lot of code worse.
I think that can be tweaked, e. g. like rubocop styling rules.
Any linter that would tell me to sprinkle my sane code via &nil
I'd throw out the door though. ;)
zverok wrote:
OTOH, would help a lot (and I am not sure why is it
closed, no reasons are
given at
https://bugs.ruby-lang.org/issues/15554?tab=history#note-9
That's from 5 years ago or so. Perhaps it may be useful to have
it at an upcoming developer meeting, discussed, and then also
someone who feels motivated and has time, to explain the decision
making towards pro or con of it anyway, since that seems to be
missing slightly (or, at the least, the context has not been put
down in detail; ufuk here may have given the biggest summary so
far about it; also note that ko1 in #15554 also mentioned a warning
rather than the raise ArgumentError as pointed out by eregon. I
guess it comes down to how you want to treat block variables in
general, which is a design consideration of ruby.)
----------------------------------------
Feature #19979: Allow methods to declare that they don't accept a block via
`&nil`
https://bugs.ruby-lang.org/issues/19979#change-105116
* Author: ufuk (Ufuk Kayserilioglu)
* Status: Open
* Priority: Normal
----------------------------------------
## 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/b4357853c03dfe71b6…,
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.
--
https://bugs.ruby-lang.org/