From: Ruby-Lang@...
Date: 2018-10-28T09:10:04+00:00
Subject: [ruby-core:89599] [Ruby trunk Feature#14145] Proposal: Better	Method#inspect

Issue #14145 has been updated by jwmittag (J��rg W Mittag).


guilhermereiscampos (Guilherme Reis Campos) wrote:
> zverok (Victor Shepelev) wrote:
> > ```ruby
> > # We can't extract default values, but at least we can say they are there
> > Addressable::URI.method(:heuristic_parse)
> > # => #<Method Addressable::URI.heuristic_parse(uri, hints = <default>)>
> > ```
> I wonder why is not possible to extract the default values?

Because they can be arbitrary Ruby code and Ruby is a Turing-complete language, which means that figuring out statically what the default value is, is equivalent to solving the Halting Problem. Additionally, Ruby is also capable of I/O and other side-effects, which means that the value can depend on external entities not visible to the documentation generator.

What would `Method#inspect` show for these:

```ruby
def foo(a = while true do; end) end

def bar(a = Time.now) end

def baz(a = if rand < 0.5 then 23 else 'fourty-two' end) end

def qux(a = File.read('somefile.txt')) end
```

And what about this:

```ruby
def crazy(a = FileUtils.rm_rf('/')) end
```

Would you expect `Method#inspect` to evaluate the default argument in order to be able to display its value?

Also, `Method#inspect` returns a `String`, but that loses all information about the type and structure of the default value:

```ruby
class Foo
  def inspect; '42' end
end

def foo(a = Foo.new) end

def bar(a = 42) end

method(:foo).inspect
#=> #<Method: Object#foo(a = 42)>

method(:bar).inspect
#=> #<Method: Object#bar(a = 42)>
```

We could copy the source text used to define the default argument into the output of `Method#inspect`, but that would require the source code to be available at runtime, which is a massive memory overhead (one `String` for every optional parameter in the entire system, and there is no upper bound on the size of that `String`, since a default argument can be any arbitrarily large Ruby program). Plus, there is the additional complication that not all methods in a Ruby system even *have* (Ruby) source code.

----------------------------------------
Feature #14145: Proposal: Better Method#inspect
https://bugs.ruby-lang.org/issues/14145#change-74637

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
The idea: When investigating (in example scripts, debugger or console) the library you are unfamiliar with, Ruby's reflection is very useful mechanism to understand "what it can": classes, modules, their constants, methods and so on.
I propose to expose a bit more information Ruby has internally in `Method#inspect`:


```ruby
# before:
some_interesting_object.method(:foo) # => #<Method Klass#foo>
# after:
some_interesting_object.method(:foo) # => #<Method Klass#foo(first_arg, *other_args, keyword_arg:)>
```

Dead-naive implementation:

```ruby
class Method
  def signature
    recv = case receiver
    when Module
      "#{receiver.name}."
    else
      "#{receiver.class}#"
    end
    parameters.map.with_index { |(type, name), i|
      case type
      when :req then "#{name || "param#{i+1}"}"
      when :opt then "#{name || "param#{i+1}"} = <default>"
      when :keyreq then "#{name || "kw#{i+1}"}:"
      when :key then "#{name || "kwparam#{i+1}"}: <default>"
      when :rest then "*#{name || "rest"}"
      when :keyrest then "**#{name || "kwrest"}"
      end
    }.join(', ').prepend("#{recv}#{name}(") << ")"
  end

  def inspect
    "#<#{self.class.name} #{signature}>"
  end
end

```

This works "sub-optimal" for methods implemented in C, yet pretty decently for Ruby-implemented methods:

```ruby
# C method, default param names
[1,2,3].method(:at)
# => #<Method Array#at(param1)>

# Ruby method, proper param names
CGI.method(:escape)
# => #<Method CGI.escape(string)>
Addressable::URI.method(:parse)
# => #<Method Addressable::URI.parse(uri)>
Addressable::URI.method(:join)
 => #<Method Addressable::URI.join(*uris)>

# We can't extract default values, but at least we can say they are there
Addressable::URI.method(:heuristic_parse)
# => #<Method Addressable::URI.heuristic_parse(uri, hints = <default>)>
```

If the proposal is accepted, I am ready to implement it properly in C (for all callable objects: `Method`, `UnboundMethod`, `Proc`)



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