Issue #14904 has been updated by jeremyevans0 (Jeremy Evans).


prijutme4ty (Ilya Vorontsov) wrote:
> My use case is just a simple example showing why current behavior isn't as good as it can be.
> 
> That's not about lambdas at all. That's about instance_eval being more clever, because there's no reason to yield receiver to a block when block can't handle it. It doesn't make lambda functions weaker, just about correct use of blocks of different arity by instance_eval.

One problem here is the slippery slope.  There are probably many core methods that take blocks where arguments are yielded, where potentially you could use them with zero argument lambdas.  `Integer#times` comes to mind as the most obvious example.  Having all these methods do block arity checking and changing what they yield based on the arity leads to added complexity, and I don't think adding such complexity is a good tradeoff.  Especially in this case where you can just use `instance_exec`.

----------------------------------------
Feature #14904: Make it possible to run instance_eval with zero-arguments lambda
https://bugs.ruby-lang.org/issues/14904#change-72911

* Author: prijutme4ty (Ilya Vorontsov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
At a moment #instance_eval always yields an object to a given block. Though if we passed lambda as a block for instance_eval, its work depend on number of arguments. But I see no reason for such strictness when lambda gets zero arguments. In that case instance_eval can just skip yielding receiver to its block.
It will reduce surprise for a newcomers who don't know the difference between proc and lambda.

I got to this problem with code similar to this:

```
module ConfigParams
  def add_config_parameter(param_name, **kwargs, &block)
    # ...
    define_method(param_name){ kwargs[:default].call(self) }
  end
end
class KeyBindings
  extend ConfigParams
  add_config_parameter :undo
  add_config_parameter :redo, default: ->{ "shift+#{undo}" }
end
kb = KeyBindings.new
kb.undo = 'ctrl+z'
puts kb.redo # => shift+ctrl+z
kb.redo = 'ctrl+y'
```

I want to allow user to express defaults with lambda which will be later evaluated in the context of an instance. It's a bit more readable than:
```
add_config_parameter :redo, default: proc{ "shift+#{undo}" }
```
And anyway, it'd be good if a user preferred to change proc into lambda didn't get strange exceptions.

I've already found different solution - to use instance_exec instead of instance_eval. But I'm still sure that life could be easier it instance_eval handled zero-argument lambdas properly.

Related: #10513



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