blocks

[Jecel Assumpcao Jr]

On Thursday 20 November 2003 07:17, Mike Austin wrote:
> --- In self-interest at yahoogroups.com, Jecel Assumpcao Jr <jecel at m...>
>
> wrote:
> > But what is "their enclosing environment"?
>
> B) The activation object, or method-instance for another term

Good.

> > And who are we talking about in "their proto (parent, actually) set
>
> to"?
>
> >   1) The block literal object?
> >   2) The result of referencing 1 with a "push literal"?
> >   3) The block closure resulting from sending 'value' to 2?
>
> 1) The block literal object

Now we have a problem. Unfortunately, B doesn't yet exist when the 
parser is creating 1 from the source text. It is likely that you have 
different definitions from what I am used to, so I will try to be 
clearer below.

> Although Io handles locals a bit different, it is essentially the
> same idea when I do this:
>
> myObject = Object clone do (
> ` add = method ( n,
> ` ` return block( x, return n + x )

The expression "block(...)" is what I called 1 above.


> ` )
> )
>
> a = myObject add( 10 )
> b = myObject add( 20 )

a and b are two instances of what I called 2 above.

> write( a( 10 ), "\n" )
> write( b( 10 ), "\n" )

a( 10 ) and b( 10 ) are two instances of what I called 3 above.

> > io closure_test.io
>
> 20
> 30
>
> The returned block's proto is set to the new "Locals" instance that
> is created when calling add.  It works simply, and beautifully.

Ok, so you set the proto of 2 to B and then 3 gets to inherit it. I 
haven't paid attention to such details in Io, however, and would be 
happy for you or Steve to correct me.

It certainly does work, but I wouldn't call it simple. Since Self 
doesn't allow non-lifo blocks, I'll rewrite your example in Smalltalk 
instead:

MyObject>>add: n
  ^ [:x | x + n]

a := myObject add: 10.
b := myObject add: 20.

Transcript show: (a value: 10).
Transcript show: (b value: 10).

I will agree that if you don't care about the multiple steps and objects 
needed to make this work, it looks simple enough. Explaining how to use 
blocks isn't a problem for me, but explaining how they get their job 
done is.

-- Jecel