From: MonkeeSage <MonkeeSage@...>
Date: 2007-12-07T09:30:04+09:00
Subject: Re: Worth an RCR? static_type_check, polymorphic_type_check, 	quacks_like

On Dec 6, 5:16 am, Robert Klemme <shortcut...@googlemail.com> wrote:
> 2007/12/6, MonkeeSage <MonkeeS...@gmail.com>:
>
>
>
> > On Dec 5, 7:08 pm, Trans <transf...@gmail.com> wrote:
> > > On Dec 5, 6:48 pm, John Carter <john.car...@tait.co.nz> wrote:
>
> > > > Is there another library like this? I would love it if it were just
> > > > standard methods on Object.
>
> > > > I find this very useful and 'require' it into all code I write. Just
> > > > helps me mop up those problems that static type checking would have
> > > > found for me...
>
> > > > Heres a typical usage, create an object and initialize it with
> > > > stuff. Only much later when you invoke the methods on the object do
> > > > you find you initialized it with the wrong stuff.
>
> > > > Got the parameter order wrong or something. But then the bug isn't on
> > > > the backtrace. So put when you initialize your instance variables you
> > > > can type check'em like so...
>
> > > > eg.
> > > > class MyObject
> > > >    def initialize( _name, _drawable, _theme)
> > > >      @name     = _name.static_type_check String
> > > >      @drawable = _drawable.polymorphic_type_check Drawable
> > > >      @theme    = _theme.quacks_like :border_color, :fill_colour
> > > >    end
>
> > > Special methods? Just do:
>
> > >     def initialize( _name, _drawable, _theme)
> > >       raise ArgumentError unless _name.instance_of?(String)
> > >       raise ArgumentError unless _drawable.kind_of?(Drawable)
> > >       raise ArgumentError unless [:border_color, :fill_color].all?{|m|
> > > _theme.respond_to?(m)}
> > >       @name, @drawable, @theme = _name, _drawable, _theme
> > >     end
>
> > > Besides, with exception to the 2nd (done for the right reason) this is
> > > all generally considered a bad practice. If you want to do it for
> > > debugging purposes that's cool --wrap the the check in an 'if $DEBUG'.
> > > But the idea behind duck-typing is to NOT do these types of checks.
> > > Just let it ride, so we can drop whatever we want into your method --
> > > if our object is "adapted" right then it can work regardless.
>
> > > Note: I used to think the last made sense. But I've learned over time
> > > that it actually is worse --you're assuming those methods dictate some
> > > type of behavior, but those methods might not do what you think they
> > > do. Use of respond_to? should be generally avoided.
>
> > > T.
>
> > Heh. I used to think the opposite about respond_to?, but for a while
> > I've been thinking about it as not asking for a promise of behavior,
> > but for a promise that it's safe to act like we have a promise of
> > behavior. Just like when we use "begin...rescue NoMethodError", we're
> > not saying "now I've ensured X object will behave well", we're saying
> > "now I can safely pretend like object X implements some interface" The
> > only difference *at all* that I see between asking respond_to? and
> > using a "begin...rescue NoMethodError" clause, is that one asks for
> > permission and the other asks for forgiveness (i.e., the only
> > difference is the point in time at which they occur). That said, there
> > are problems with method signature validation by respond_to?, as many
> > people have pointed out before, e.g., method_missing dispatching with
> > variable arity. So, while I would agree that it may not often be a
> > good idea, pragmatically, I disagree with the idea that it
> > *necessarily* promotes a wrong way of thinking about programming (or
> > at least programming in ruby).
>
> Some more random thoughts about respond_to?
>
> 1. the point in time may be crucial, i.e. when checking on
> initialization the object might actually not yet respond to the method
> but logic otherwise guarantees that it does when the method is
> actually invoked.
>
> 2. point in time is not the only difference as you have pointed out
> (method_missing).
>
> 3. the net effect is the same, i.e. if you check with respond_to? you
> will have to raise an exception - you will get an exception as well
> when you simply call the method.

I can't think of an actual use case offhand (heh, that probably speaks
for itself), but I've used it before in places where the exceptional
case is non-terminal. A contrived example...

class B < Array; private :index; end
b = B.new
unless b.respond_to? :index
  p "state 1, use #foo"
  class B; public :index; end
end
#...
if b.respond_to? :index
  p "state 2, use #index"
  class B; private :index; end
end

> For the me disadvantages of respond_to? and type checks by far
> outweigh the benefits (assumed increase in robustness or faster bug
> finding).
>
> Kind regards
>
> robert
>
> --
> use.inject do |as, often| as.you_can - without end

Regards,
Jordan