Re: Proposed change to BC iterator parameters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"amado.alves" <amado.alves@netcabo.pt> writes:

> "...Ada has such weak support for iterators."
>  
> Will you stop saying that?

Sorry, I thought this newsgroup was for discussing Ada.
I didn't realize it was purely a cheerleading-for-Ada club.  ;-)

>  An iterator is an abstraction. No general purpose language is suppose
>  to have "support for iterators". Ada provides many building blocks
>  appropriate for constructing iterators.

I've nothing against the "building block" approach.  The problem is that
Ada doesn't have the right building blocks to support iterators.

Some general-purpose languages, by the way, *do* have direct support for
iterators.  CLU and Sather come to mind.  Other languages have
reasonable building blocks.  Lisp and Smalltalk come to mind (although
they lose compile-time type checking).  Even Pascal has better building
blocks (for iterators) than Ada -- Pascal has downward closures.

> The only thing lacking is
>  perhaps anonymous subprograms (e.g. like blocks in Smalltalk). But I
>  fear adding that would go against a number of the "-ilities".

No, that's far from the "only" thing lacking.  I mentioned several
others in other posts.  The main one is downward closures.

I don't know of any "-ilities" that anonymous subprograms would
necessarily violate; I think that's a FUD argument.  Of course, we
wouldn't want to do it exactly like Smalltalk, where the parameter types
are unknown.

The current building blocks violate at least one important "ility":
readability.  You have to instantiate separately, and you have to split
the loop body out into a separate procedure, and you have to pass extra
paraameters around, all of which means that the pieces of the loop get
scattered about.  And you have to clutter the namespace with a name for
the loop body, which is not (usually) a natural abstraction on its own,
and doesn't deserve a separate name.  Or you have to use the "cursor"
style, which has problems I pointed out elsewhere.

- Bob