From: Albert Wagner <alwagner@...>
Date: 2002-01-20T03:45:03+09:00
Subject: Re: dynamic method creation

On Saturday 19 January 2002 06:33 am, you wrote:
> On Sat, Jan 19, 2002 at 04:32:43AM +0900, Albert L. Wagner wrote:
> > I have a need to dynamically create methods with method names
> > that are not known until run time.  Below is my current best
> > shot.  Is there a better way? If methods allowed expressions as
> > names, and not just as qualifiers, I wouldn't need the eval.
> >
> > class Record
> > 	def initialize(name)
> > 		@name = name
> > 	end
> > end
> >
> > fieldNames = ['abc', 'dof', 'ghi']
> > fieldTypes = ['String', 'Fixnum', 'Float']
> > rec = Record.new('aRecord')
> > class << rec
> > end
> > fieldNames.size.times do | i |
> > 	f = fieldNames[i]
> > 	defStr = "def rec.#{f}=(obj)\n"
> > 	case fieldTypes[i]
> > 		when 'String'		then defStr << "@#{f} = obj\n"
> > 		when 'Fixnum'		then defStr << "@#{f} = obj\n"
> > 		when 'Float'		then defStr << "@#{f} = obj\n"
> > 	end
> > 	defStr << "end\n"
> > 	puts defStr
> > 	eval defStr
> > end
> > p rec.abc=(nil)
> > p rec.dof=(nil)
> > p rec.ghi=(nil)
> > p rec.methods
>
> To do what your example does, I would just write:
>
> Record = Struct.new(nil, :abc, :dof, :ghi)
> rec = Record.new('one', 2, 3.0)
>
> p rec.abc
> p rec.dof
> p rec.ghi
>

Yes, That is how I originally implemented the problem.  It's clean and 
simple.  I like Structs conceptually.  But there turned out to be some issues 
with Struct and Marshal that I didn't understand, and once I lose faith in a 
mechanism, then it's hard to rely on it again until a period of 
"forgetfulness" passes.  Also, I discovered that I wanted more functionality 
than Struct offered.

>
> To do what your question asks, in the past I've written:
>
> class Triangle
>   def initialize
>     @variables = {
>
>       :area => 0,
>       :base => 0,
>       :height => 0
>
>     }
>   end
>
>   def method_missing(id, *args, &block)
>     id.to_s =~ /(\w+)(=)*/
>     id, assign = $1.intern, $2
>
>     if @variables.include?(id)
>       if assign
> 	return @variables[id] = args.shift
>       else
> 	return @variables[id]
>       end
>     else
>       super
>     end
>   end
> end
>
> t = Triangle.new
>
> t.area = 10
> t.base = 5
> p t.area
> p t.base
>

I haven't yet worked with method_missing, which is peculiar as I come from 
smalltalk.

>
> The @variables hash can, of course, be made accessible from outside
> the class to do what you want to do.  However, this doesn't get more
> useful than a Struct.  Unless...
>
> class SolvableTriangle < Triangle
>   def initialize
>     super
>
>     @formulas = {
>
>       :area => proc { base * height / 2.0 },
>       :base => proc { area * 2 / base.to_f },
>       :height => proc { area * 2 / height.to_f}
>
>     }
>   end
>
>   def calc(id)
>     @variables[id] = @formulas[id].call
>   end
> end
>
> st = SolvableTriangle.new
>
> st.base = 5
> st.height = 4
> st.calc :area
> p st.area
>
>
> Lastly, I do most of the above (defining variables *and* their
> relations) through including a simple module that accepts one or more
> equations, extracts variables, and through YACAS (a symbolic algebra
> program) solves the equation(s) for each of them.
>
> class Triangle
>   include Equation
>
>   def initialize
>     defeq 'area==base*height/2'
>     compile
>   end
> end
>
> class Equilateral < Triangle
>   def initialize
>     defeq 'height==side*0.866'
>     super
>   end
> end
>
>
> t = Equilateral.new
>
> t.side = 5
> t.calc :height, [:side]
>
> t.base = 4
> t.area = 10
> t.calc :height, [:base, :area]
>
>
> As above, nothing prevents from defining the equations from outside.
>
> (Yep, this is more than what you asked, but I hope you enjoyed the
> overview.)

Yes, thank you.  I did enjoy it and, as always, I learn from it.

>
>
> Massimiliano

-- 
Quantum Mechanics: The dreams stuff is made of