From: Gregory Seidman <gsslist+ruby@...>
Date: 2007-05-26T02:59:33+09:00
Subject: Re: Introducing the "it" keyword

On Fri, May 25, 2007 at 04:58:08PM +0900, Ronald Fischer wrote:
> > This is obviously suboptimal code, as is, because it results in the
> > evaluation of m(x) twice. I propose a new keyword is added, "it",
> > which may appear within the statement to the left of the decorator.
> > So, the previous statements become:
> > 
> > puts "User: #{it}" if opts[:user]
> > 
> > and
> > 
> > return it if |v + 1| < 10
> 
> IMO, this is a special case of the more general idea that within a
> certain piece of code, the value of an expression needs to be refered to
> more offen.
> 
> Looking at how others do this, the functional programming / Scheme people
> usually handle it with a 'let' construct:
> 
>   let v = opts[:user] in 
>     puts "User: #{v}" if v
> 
> Of course this has the disadvantage that we *have* to invent a new name
> (v). But assuming that such a substition is only done locally, we could
> as well use placeholders. 'it' would be such a placeholder, kind of:
> 
>   let opts[:user] in puts "User: #{it}" if it
> 
> but this would restrict us to have only one binding at any time. A more
> general solution could be like this:
> 
>   let a+b+c in
>     let foo=d*e in
>       let x/y in
>         puts ($1+foo)*($1-$2)-$2/($1-foo)
> 
> Here, $1 would be bound to x/y (innermost unnamed let binding), $2 would
> be bound to a+b+c (second innermost unnamed let binding). This is similar
> to the binding of parameters in the DeBrujin Lambda calculus, if my rusty
> memory is right (any FP people here who can comment on this?).
[...]

You're working too hard:

def let(*args)
  yield *args
end

def use_let(a,b,c)
  let a+b+c do |it|
    return it if it < 10
  end
end

use_let(1,2,3) # 6
use_let(4,5,6) # nil

No need for additional keywords or even global variables, just a little
method named let.

> Ronald
--Greg