Feature #15357
Updated by larskanis (Lars Kanis) almost 6 years ago
The current implementation of Proc#parameters [differentiate between lambda? true and false](https://github.com/ruby/ruby/blob/49cd16bfaf4f03885058ce748119bc8ea2de735a/iseq.c#L2800).
This is presumably due to the fact, that [procs use tricks](https://ruby-doc.org/core-2.5.3/Proc.html#method-i-parameters) to apply arguments differently than lambda and methods.
Unfortunately `proc{}.parameters` states all `:req` parameters as `:opt`, so that these both types of parameters are not distinguishable.
This information loss leads to the situation that two different proc signatures return an equal parameters list, but behave differently:
```ruby
pr = proc{|a,b=2| [a,b] }
pr.parameters # => [[:opt, :a], [:opt, :b]]
pr.call(:a) # => [:a, 2] # :a is interpret as the first parameter
pr = proc{|a=1,b| [a,b] }
pr.parameters # => [[:opt, :a], [:opt, :b]]
pr.call(:a) # => [1, :a] # :a is interpret as the second parameter
```
That means that the current return values of `proc{}.parameters` are not suitable to build wrapper or proxy objects for a proc.
In Eventbox a I use the workaround is used: The to pass the proc is passed to `define_method` and [query the [method method parameters are retrieved instead](https://github.com/larskanis/eventbox/blob/3bcbc30096c6003e96d41c6496c781dfc90ac36a/lib/eventbox/argument_wrapper.rb#L10-L17).
That way the list of parameters can be retrieved including `:req` and `:opt` differentiation, so that a wrapper proc can be built.
The application of argument assignment tricks is are a property of the Proc object - object, but not a property of single parameters. Therefore it shouldn't be applied to `Proc#parameters` in addition - at least not in a way that discards valuable information.
The property of the Proc object is already readable through `Proc#lambda?`, so that there's no need to apply this property to `Proc#parameters` as well.
My proposal is to unify `proc{}.parameters` and `lambda{}.parameters`, so that `proc{}.parameters` shows positional returns `:req` arguments without default value as type `:req` as well.