From: Ronald Fischer <ronald.fischer@...>
Date: 2007-05-25T16:58:08+09:00
Subject: Re: Introducing the "it" keyword


> 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?).

Of course it does not mean that the notation $n is really good here (as
it is
already taken by a different usage), but one could equally well think of
other
ways to denote this, such as _1 or \1 or whatever it is seen practical.

Ronald
-- 
Ronald Fischer <ronald.fischer@venyon.com>
Phone: +49-89-452133-162