[ruby-core:59710] [ruby-trunk - Bug #9397] Lambda#=== raises `ArgumentError` if the lambda accepts 0 args or requires more than 1

Issue #9397 has been updated by Benoit Daloze.


Myron Marston wrote:
> Why is it desirable for a built-in type to raise `ArgumentError` for `===` rather than returning false?

I guess mostly because it would be hard to debug why you never get in a specific clause of the case.

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Bug #9397: Lambda#=== raises `ArgumentError` if the lambda accepts 0 args or requires more than 1
https://bugs.ruby-lang.org/issues/9397#change-44227

* Author: Myron Marston
* Status: Rejected
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: 1.9+
* Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN, 2.1: UNKNOWN
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Ruby 1.9 introduced `===` on lambdas/procs so that you can put them in case statements.  I'm a fan of this, but there's an unfortunate side effect I've run into recently.

If you have a lambda that accepts 0 args (e.g. `lambda { }`) or requires more than 1 arg (e.g. `lambda { |x, y| }`), calling `===` on it will raise an `ArgumentError`.  I understand why: lambdas are strict about the number of arguments, and `===` is just an alias of `call` now.  However, this makes things difficult in a gem when you want to use `===` to match arbitrary objects (since it's the general-purpose protocol ruby provides for that purpose) and there may be lambdas provided by the user.  I ran into this in RSpec recently, and my (hacky) solution is to `rescue ArgumentError`:

https://github.com/rspec/rspec-support/commit/caf76c5de26ea1ca93f09f1097a0092ee4bf828d

It would be nice not to have to do this, and IMO, `===` on all built-in types should maintain the contract that they can be called with 1 arg and will not raise an error.  Consider that ruby's syntax doesn't even allow you to call `===` with 0 or more than 1 argument unless you resort to hacks like `send` (e.g. `obj.send(:===, x, y, z)`).  Given that, I think that `===` on lambdas should return false rather than raising an ArgumentError if the lambda does not support being called with 1 argument.





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