From: mikeharder@...
Date: 2006-10-31T19:25:06+09:00
Subject: Re: Duck typing and the Standard Library


Robert Klemme wrote:
> On 31.10.2006 10:19, mikeharder@gmail.com wrote:
> > I'm new to Ruby, so please excuse any ignorance on my part.  I read the
> > following article about Ruby and "duck typing":
> >
> > http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/100511
> >
> > I got the impression that duck typing is the "right" way to do things
> > in Ruby.  However, the Ruby Standard Library itself doesn't seem to use
> > duck typing.  Consider the following example:
> >
> > irb(main):001:0> require 'set'
> > => true
> > irb(main):002:0> s = Set.new
> > => #<Set: {}>
> > irb(main):003:0> s.superset? 0
> > ArgumentError: value must be a set
> >
> > It seems like the "superset?" method explicitly checks that its
> > parameter is a set.  This is approach #1 in the "duck typing" article
> > above, which the article claims is not "the Ruby way".
> >
> > So, what gives? If it's wrong to "try to make Ruby do Static Typing"
> > (as the article says), then why does the Ruby Standard Library do it?
>
> It is just giving you a nicer error message.  Otherwise, this might happen:
>
> irb(main):003:0> def foo(s) s=s.to_set end
> => nil
> irb(main):004:0> foo []
> => #<Set: {}>
> irb(main):005:0> foo Set.new
> => #<Set: {}>
> irb(main):006:0> foo 0
> NoMethodError: undefined method `to_set' for 0:Fixnum
>          from (irb):3:in `foo'
>          from (irb):6
>
> Note that there must be an exception of some kind if the parameter is
> inappropriate.  And no, this is not static typing.
>
> Kind regards
>
> 	robert

Yes, there must be an exception of some kind if the parameter is
inappropriate.  The question is whether to explicitly check the type of
the parameter and throw a nicer exception, or to explicitly *not* check
the type of the parameter and let the exception happen where it may
(i.e. when a needed method does not exist).

Arguments can be made for both sides, but which is considered the "best
practice" for Ruby development?  The Ruby Standard Library suggests it
is the former, but the article I referenced above suggests it is the
latter.

-Mike