From: timr <timrandg@...>
Date: 2010-11-12T10:20:15+09:00
Subject: Scheme's (cond ((assertion) (value))...(else (value))) statement implemented in ruby?

Hi Rubyists,
I love ruby, but am learning scheme and kinda like the (cond
((assertion) (value))...(else (value))) syntax for what would normally
look like this in ruby:

if (assertion 1)
  value 1
elsif (assertion 2)
  value 2
else
  default
end

I find scheme's cond syntax more quickly digestible since it reads
like a table.
(cond
    ((assertion 1) (value 1))
    ((assertion 2) (value 2))
                (else (default)))


So I wondered if I could get a similar looking function in ruby. Here
is an attempt:

class Object
  alias l lambda

  def cond(tests)
    #acts similar to scheme/LISP cond function using hash
    #key => value rather than scheme's (cond (assertion)(value)...
(else(value)))
    tests.each_pair do |k,v|
      if k.call
        v.call
        break
      else
        next
      end
    end
  end

  def end_cond; l{true}; end
end

cond  l{3 < 2}      =>  l{ puts "3 is less than 2"},
         l{3 > 3}      =>  l{ puts "3 is greater than 3"},
         l{3+1 > 4}  =>  l{ puts "4 is greater than 4"},
       end_cond    =>  l{ puts "hey, it works!"}

# >> hey, it works!

I was surprised that the hash seems not to mix-up the order of the k:v
pairs like it would if the keys were strings or symbols. I suppose
because they are lambda's, the hash doesn't bother alphabetizing them
during for the .each_pair method. Since they stay ordered, this table-
like if;elsif;else seems to work fine. Can anyone think of any issues
to using this approach? Can it give rise to unpredicted values--due to
some behind the curtain hash key sorting? Or can anyone come up with a
cleaner syntax than a hash of lambdas for both keys and values? I know
this is not idiomatic ruby here. I am just having fun.