From: Eric Mahurin <eric_mahurin@...>
Date: 2005-09-14T01:53:00+09:00
Subject: Re: yet another simple command-line option parser

Thanks for the input Jim.  Comments below.  I'm also putting
this in the RCR comments.

--- Jim Freeze <jim@freeze.org> wrote:
> That's pretty interesting Eric, to grab the type off the
> default.
> I think I'll add that to CommandLine::OptionParser.
> 
> However, I'm still not sure if I like the #from_s form, 
> but I can see the utility of it. For the common cases, 
> I can use a simple case statement:
> 
>   case default
>     when Float then Float(arg)
>     when Fixnum then Integer(arg)
>   end
> 
> But, as you can see with even these simple cases, there
> are big issues and big questions to answer.
> 1. Fixnum does not match Integer

No problem.  Fixnum inherits from Integer.  Do you care whether
Fixnum.from_s returns a Fixnum or a Bignum?  It could return
either just like many of the other Fixnum instance methods.

> 2. Do we use to_i or Integer(#) - Integer raises and to_i
> does not
> 3. Do we use Float or to_f - Float raises and to_f does not

Good point.  This RCR should to specify this.  I would think it
best if an exception occur if the full string doesn't parse the
the target type.  I'll change the implementation to use the
methods that raise exceptions.

> Then, there are the tougher cases like
> 
>   require 'parsedate'
>   case default
>     when Time then Time.gm(*ParseDate.parsedate(arg))

I haven't dealt with dates and times to know what all the
options are.  I threw this in at the last minute.  If you don't
like the klass.from_s method, you could define your own derived
class (or override that klass.from_s):

require 'parsedate'
class MyTime < Time
  def self.from_s(s)
    gm(*ParseDate.parsedate(s))
  end
end

and then make the default be a MyTime instead of a Time.

>     when Fixnum arg.to_i  # what if yield bignum

answered above

>   end
> 
> where we have to use a help class and helper method (not new)
> to get
> the object we want. Or if the conversion method
> returns something other than what we requested, like
> Bignum instead of Fixnum.
> 
> Sadly, the #from_s RCR doesn't seem to address any of these
> issues.
> 
> 
> > ARGV.replace(%w(
> >     -n 4
> >     -multiplier 3.14
> >     -q
> >     -title foobar
> >     -pattern fo+
> >     -time 5:55PM
> >     -method downcase
> >     a b c
> > ))
> > # option => default (or false for a flag)
> > argv_options(
> >     :n => 1,
> >     :multiplier => 1.0,
> >     :q => false,
> >     :title => "hello world!",
> >     :pattern => /.*/,
> >     :time => Time.new,
> >     :method => :to_s
> > )
> 
> -- 
> Jim Freeze
> 
> 



	
		
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