Feature #9428


Inline argument expressions and re-assignment

Added by wardrop (Tom Wardrop) over 8 years ago. Updated over 8 years ago.

Target version:


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

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

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.

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

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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