Issue #9428 has been updated by Tom Wardrop.


> "First encountered" in regular left-to-right parsing order?

It would be first identifier encountered as per the order of execution. In the following example, the variable in the if statement would be the name of the argument.

    def foo(id.to_i if String === bob )

You could rewrite this as...

    def foo(bob)
      bob = id.to_i if String === bob
    end

A contrived and fairly non-sensical example, but it demonstrates that the variable furthest to the left isn't necessarily the variable name.

> This introduces some amount of confusion. Which of the following is
equivalent?

The first case `id = id.to_i // id = config[:default_id]`. If an argument is given `id = id.to_i`, if the argument is omitted, the argument name would be inferred from the expression on the left (`id.to_i`) and the result of  `config[:default_id]` would be assigned to it as in `id = config[:default_id]`. I'm not suggesting anyone would want to do this, but it's possible.

As Matz has indicated though, it would be very difficult to parse. Fun to discuss though.

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Feature #9428: Inline argument expressions and re-assignment
https://bugs.ruby-lang.org/issues/9428#change-44440

* Author: Tom Wardrop
* Status: Rejected
* Priority: Normal
* Assignee: 
* Category: core
* Target version: 
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Just a random idea. Currently, Ruby allows you to use any arbitrary expression for setting default values for arguments, which can be really convenient and makes for clear code, especially handy for documentation, etc. For example:

    def fetch(id, cache = config[:cache])
      # bleh
    end

In the same vein, as well as setting a default value using an arbitrary expression, it's not uncommon to *post-process* an argument, some common examples include:

    arg = arg.upcase
    arg = arg.to_sym
    arg = arg.dup

It would be rather nice in my opinion to be able to do this inline when defining the argument:

    def fetch(id.to_i, cache = config[:cache])
      # bleh
    end

This works well where the argument is the receiver of the method call, but what if you wanted to do `Integer(id)` in the above example instead of using String#to_i? There are two options. One could either fallback to processing the argument within the method/block body, or, you could make the implementation a little bit clever by using inferencing.

Ruby could auto-assign the passed argument to the first variable encountered in the expression. So in the following example, as soon as the virtual machine encounters `id`, it recognises it as a variable and assigns the argument value before continuing. When encountering subsequent variables, Ruby would take the usual action and look for a corresponding method in `self` before throwing an error. You can always disambiguate by qualifying the receiver, e.g. `self.id`

    def fetch(Integer(id), cache = config[:cache])
      # bleh
    end

Whatever the result of the expression, it's assigned as the final argument value. So in the case of `id.to_i`, the argument name of `id` is inferred. `id` is set to the supplied argument for the duration of the expression. The result of the expression is then re-assigned as the value of `id`. This technically allows expressions of arbitrary complexity, but like all things in Ruby, with great power comes great responsibility. One must use common sense when deciding whether to manipulate the argument inline, or within the method body. As long as the expression is of reasonable length and complexity, readability remains perfectly reasonable.

Interested to get some thoughts and opinions on this one. I sense the potential for controversy :)




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