From: Leif Eriksen <leif.eriksen@...>
Date: 2008-01-31T09:37:53+09:00
Subject: Idiomatic way to copy parameters ?

Some background - bear with me

So I'm writing my first little library (module) in ruby, for a 
play-project in rails.

The library is for color management related functions, and its first 
method is to
provide a color 'gradient', which I use for images where the colors smoothly
change from a to b in as many steps as I require

for example, to get from html rgb color #010101 to #444444 in 5 'steps', 
we would do

ColorManagement.gradient(#010101, #444444, 5) # will used named params 
in version 2!

and it would return

["#010101", "#0e0e0e", "#1b1b1b", "#282828", "#353535", "#444444"]

The method supports leading '0x' chars as well, and prepends the leading 
chars (if any) to the
entries in the result array.

And this is all cool, until I did this

start = '#010101'
finish = '#444444'
ColorGradient.gradient(start, finish , 5)
pp start "#{start}"
pp "finish #{finish }"

and I see this

"start 010101"
"finish 444444"

The '#''s are gone. Internally the library is stripping these off to make
splitting into RGB channels easier, and it does the stripping like this

  def self.gradient(hex_start="000000", hex_end="FFFFFF", steps=256)
      [hex_start,hex_end].each do |hex|
         hex.sub!(/^(0[xX]|#)/) do |match|
           match = '' # remove any leading # or Ox
 
So the strings that are passed in are permanently munged by the sub! . 
OK I get that, and to solve it I did this

  def self.gradient(param_start="000000", param_end="FFFFFF", steps=256)
    #make local copies of parameters
    hex_start = String.new(param_start)
    hex_end   = String.new(param_end)
   
      [hex_start,hex_end].each do |hex|
        hex.sub!(/^(0[xX]|#)/) do |match|
        ...

So in effect I copy the parameter strings into local vars, to avoid 
munging what the user passes me and annoying them.

But is this the ruby way ? Is this the idiomatic way to avoid 
side-effects on objects passed in as parameters ?

Cheers
Leif