From: Gennady <gfb@...>
Date: 2004-02-04T01:00:25+09:00
Subject: Re: RCR draft for enhanced "case..when..else..end" syntax

Guoliang Cao wrote:
> Hi,
> 
> I'm thinking of submitting a RCR. Here is the draft. Comments are welcome.
> 
> Thanks,
> Cao

Do you know about this syntax currently available?

a = "aaa"
b = "bbb"
case
   when a == "aaa", b == "bbb"
     puts "Lowercase detected"
   when a == "AAA", b == "BBB"
     puts "Uppercase detected"
   else
     puts "Detection failed"
end

> 
> ====================================================
> 
> RCR title:  An enhanced case..when..else..end syntax
> 
> This RCR involves (check all that apply):
> ... a new feature                   x
> ... a syntax change                 x
> ... refactoring only            
> ... backwards incompatibility
> 
> Abstract....
>   This RCR proposes an enhanced case..when..else..end syntax.
>   
> Problem....
>   It's a very common task to group code based on a combination of values of
>   multiple variables. However, the current Ruby syntax only allows one
>   expression/variable being used in "case" clause. People have to use
>   if..else..end which requires more typing and is less intuitive.
>   
>   For example, suppose I'm generating a image, the color of each point is
>   defined based on its x and y coordinates.
> 
>        0 ... 50...100...200
>      0 +----------------+
>        |      white     |
>     50 |----------------|
>        |     |  grey    |
>    100 |blue +----------|
>        |     |  black   |
>    150 |----------------|
>        |      white     |
>    200 +----------------+
> 
> 
>   Use if..else..end:
> 
>   if 0..50 === x || 150..200 === x
>     color = white
>   elsif 0..50 === y
>     color = blue
>   elsif 50..100 === x and 50..200 === y
>     color = grey
>   else
>     color = black
>   end
> 
>   Use new case..when..else..end:
>    
>   case x : y
>     when 0..50, 150..200             # x in 0..50 or 150..200
>       color = white
>     when _ : 0..50                   # y in 0..50
>       color = blue
>     when 50..100 : 50..200           # x in 50..100 and y in 50..200
>       color = grey
>     else                             # otherwise
>       color = black
>   end
> 
>   Note: If the color is only dependent on x, we can use the current "case"
>   syntax. However if it's dependent on more than one variable, "case" is
>   incapable of handling it.
> 
> Proposal....
>   The enhanced syntax looks like below:
> 
>   case expr1 : expr2
>     when expr3 : _ : expr4
>       do_something
>     when expr5
>       do_different_thing
>     else
>       do_default_thing
>   end
> 
>   1. "case" takes 0 or any number of expressions separated by ":";
>   
>   If there's no expression, each clause separated by ":" in "when" is
>   evaluated to true or false and ":" is treated the same way as "and".
> 
>   2. "when" takes 1 or more clauses separated by ":"; each clause by itself
>   can be separated by "," (this is supported in current syntax).
>   
>   If there are less clauses than the number of "case" expressions, the
>   rest of "case" expressions are not evaluated.
> 
>   If there are more clauses than the number of "case" expressions, the
>   rest of "when" clauses are evaluated to true or false. ":" is treated
>   the same way as "and"
> 
>   3. "else" remains the same.
> 
>   4. "_" means skipping evaluation of the corresponding "case" expression.
> 
> 
> Analysis....
>   What benefits do we get?
>   1. It's a more general form, which means it can be used much wider than
>   what "case..when" is used for today;
>   2. less typing; there's no need to type x, y, === in the when clause;
>   3. this syntax change is backward compatible with Ruby 1.x
>   
> Implementation....
>   Because this request proposes syntax enhancement, it can't be achieved
>   without changing Ruby interpreter.
>   
>   The code below demonstrated the idea by extending Object class and
>   utilizing thread-local variables. It uses "ccase..cwhen..celse..end" to
>   simulate the proposed syntax. It uses "," to separate ccase/cwhen clauses
>   which are just arguments. It uses "true" instead of "_" to indicate
>   skipping evaluation.
>   
> 
>   Source:
> 
> #!/usr/bin/ruby
> #
> # To utilize Ruby's existing functionality to demonstrate the idea, I'm doing
> # this:
> # add instance methods "ccase", "cwhen", "celse" to class Object which is the
> # top one in the hierarchy so all classes inherit them.
> #
> # Its usage is like this:
> #   ccase a, b do
> #     cwhen [1,2], 1..3 do
> #       ...
> #     end
> #     cwhen 3, true do
> #       ...
> #     end
> #     celse do
> #       ...
> #     end
> #   end
> #
> # Problems so far:
> # 1. how to let "cwhen" method body to access "ccase" arguments and
> #    intermediate objects;
> #    Solution: use thread-local variable
> #
> # 2. how to make it work in multi-threaded program;
> #    Solution: use thread-local variable
> #
> # 3. "do...end" or "{...}" is needed to pass block to ccase/cwhen/celse.
> #    Solution: ???
> #
> # ccase = "customized case"
> # cwhen = "customized when"
> # celse = "customized else"
> #
> 
> class Object
> 
> # customized case
> def ccase arg0, *args
>   begin
>     args.unshift arg0
>     Thread.current["in_ccase"] = true
>     Thread.current["ccase_args"] = args
>     Thread.current["hit_cwhen"] = nil
>     if block_given?
>       yield
>     end
>   ensure
>     Thread.current["in_ccase"] = nil
>     Thread.current["ccase_args"] = nil
>     Thread.current["hit_cwhen"] = nil
>   end
> end
> 
> # customized when
> def cwhen arg0, *args
>   # skip if already matched
>   return if Thread.current["hit_cwhen"]
> 
>   args.unshift arg0
>   hit = true   # hit or not?
>   ccase_args = Thread.current["ccase_args"]
>   
>   if ccase_args and ccase_args.length > 0
>     ccase_args.each_index do |idx|
>       ccase_arg = ccase_args[idx]
>       cwhen_arg = args[idx]
> 
>       # cwhen_arg is not present, hit = true
>       # cwhen_arg is true and ccase_arg is not false, hit = true
>       # ccase_arg is not present, hit = if cwhen_arg evaluates to true
>       # both ccase_arg and cwhen_arg are present, hit = if cwhen_arg === ccase_arg
>       if (not cwhen_arg) or (not ccase_arg and cwhen_arg) or (cwhen_arg == true and not ccase_arg == false) or (cwhen_arg === ccase_arg)
>         # hit = true
>       elsif cwhen_arg.is_a? Array
>         hit = false
>         cwhen_arg.each do |arg|
>           if arg === ccase_arg
>             hit = true
>             break
>           end
>         end
>       else
>         hit = false
>       end
>       s = if cwhen_arg.nil? then "nil" else cwhen_arg.to_s end
>       s1 = if ccase_arg.nil? then "nil" else ccase_arg.to_s end
>       print "#{s} === #{s1} : #{hit.to_s}\n"
> 
>       break if not hit
>     end
>   else
>     args.each {|arg| hit &= arg}
>   end
> 
>   return if not hit
>   Thread.current["hit_cwhen"] = true
> 
>   if block_given?
>     yield
>   end
> end
> 
> # customized else to be combined with ccase/cwhen
> def celse
>   return if Thread.current["hit_cwhen"]
>   if block_given?
>     yield
>   end
> end
> 
> end
> 
> if __FILE__ == $0
>   def test a,b,c
>     ccase a,b,c do
>       cwhen [1, 3, 6], 1..2 do
>         # if a is in [1,3,6]  and  b === 1..2
>         print "hit 1\n"
>       end
>       cwhen 4, 3, 1 do
>         # if a === 4  and b === 4 and c === 1
>         print "hit 2\n"
>       end
>       cwhen 5, true, 7 do
>         # if a === 5  and  c === 7
>         print "hit 3\n"
>       end
>       celse do
>         # otherwise
>         print "no hit\n"
>       end
>     end
>   end
> 
>   test 1,2,3  # hit 1
>   test 4,3,0  # no hit
>   test 4,3,1  # hit 2
>   test 5,6,7  # hit 3
> 
> end  
> 
>