From: Eric Mahurin <eric_mahurin@...>
Date: 2005-09-08T02:09:32+09:00
Subject: Re: Lexical Casts with Ruby

--- Paul Brannan <pbrannan@atdesk.com> wrote:

> On Sun, Sep 04, 2005 at 12:16:26AM +0900, R??diger Sonderfeld
> wrote:
> > Hello,
> > I want to assign some values I recive in string form to
> attributes of a
> > user specified class. The problem is that I have to lexical
> cast the
> > strings into the required type. I can't use to_i or to_f
> because the attribute
> > type can only be determined at runtime.
> > 
> > Is there a way for lexical casts between types in Ruby?
> Something like
> > convert("10", Fixnum)
> > => 10
> 
> In C++, boost::lexical_cast uses streams to convert the
> object to a
> string and then from the string to the desired type.
> 
> It is easy to do the first part in ruby; all objects have a
> to_s method.
> 
> The second part is harder.  There is no uniform mechanism for
> converting
> a string into an object.


A while back, I suggested adding some klass.from_* methods. 
For example:

def Integer.from_s(s)
  s.to_i;
end

Any class that could make one of its objects from a String
would put this method in.  You could do the same for other
classes to convert from also: from_i, from_f, etc.  Here would
be the usage:

Integer.from_s("10") # 10
10.to_s              # "10" - nothing new

As a convienence you could also add this:

class String
  def to(klass)
    klass.from_s(self)
  end
  def self.from(obj)
    obj.to_s
  end
end

so you could do:

"10".to(Integer) # 10
String.from(10)  # "10"

If this is a good solution for you, maybe an RCR for this more
general conversion mechanism is in order.



	
		
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